Time as Form: Lessons from the Bergson-Einstein Dispute

The confusion surrounding the early philosophical reception of Relativity theory can be traced back to a misconception regarding the status of “ time ” in philosophical — and possibly scientific — discourse. For all its empirical grounding in actual perception and measurement, time is neither an empirical object, nor a category in the ordinary sense. As Aristotle first acknowledged, time is not some abstract or idealized motion; as such, it cannot be reduced to a generic representation of becoming. Kant underscored that time itself is immune to change, suggesting that it is best characterized as a form whose function for understanding is to coordinate a cluster of ideas and problems pertaining to persistence and change, as well as coexistence, in accordance with the most general principles of experience. The vindication of the unity and universality of time by philosophers as far apart as Russell and Bergson stems from the conviction that such basic temporal ideas cannot easily be taken apart. The fact that time comprises a subjective or psychological element is, in that respect, a peripheral issue. Thus, Bergson ’ s “ quarrel ” with Einstein revolves around the possibility of apprehend-ing simultaneity at a distance as a sheaf or envelope of durations unfolding in real time. Neither proper time (invariant, local) nor coordinate time (frame-de-pendent, global) can properly reflect the intuition of that thick present. While Bergson strives to incorporate it back into the relativistic framework based on the experience of lived simultaneity, Whitehead formalizes it in terms of contemporaneous extended events.Yet both seek a regional understanding of the matter, in line with some contemporary philosophers of spacetime. The (in)famous twin paradox is examined in this light, along with certain critical concepts in Bergson ’ s philosophy of time. The challenge is to unpack the meaning of coexistence beyond the immediate phenomenological features of proximal co-presence.

ly say, Ithink, that it does not destroy the possibilityofcorrelating different local times and does not thereforehavesuch far-reachingphilosophical consequences as is sometimes supposed. In fact,inspiteofdifficulties as to measurement,the one all-embracing time still, I think, underlies all that physics has to saya bout motion (Russell 1914, 103 -104) The author of these lines is not Henri Bergson, but none other than Bertrand Russell, one of his most vocal and sarcastic philosophicalo pponents. In his 1914 Lowell lectures, published as OurK nowledgeo ft he External World,t he Cambridge philosopher deemed it "safe" to assume that philosophy-if not physics itself-would not need to relinquish the idea of au nique, "all-embracing" time as long as local time measurements could be dealt with in au nifieda nd consistent manner. Thiss omewhat controversial claim can stillb ef ound unchanged in the 1922 reprint edition of his book. It is onlyafew years later, in the revised edition of 1926,that Russell chose to suppress the entire paragraph, no doubtprompted in doing so by the harsh polemic thatfollowed the publication of Bergson'sessayonRelativity theory, Durée et Simultanéité.¹ In the meanwhile, TheA BC of Relativity had put thingss traight, emphasizing,i nw hat appears as ar adical doctrinal U-turn, "the collapse of the notion of one allembracing time" (Russell 1925,2 25).² Russell'si deas weren ow in line with the orthodoxv iewt hat Relativity theory in fact destroyst he possibility of singling out au niquelyd efined cosmic "now",a nd more generallyo fa chieving at otal temporalo rderingo fp oint-events in space-time.
One is left wondering whyRussell did not realize this clearlybefore the early twenties,o rw hy he chose to knowingly downplayo ne of the most far-reaching philosophical implications of Relativity theory and emphasize instead what may seem, under the most charitable interpretation,arather trivial point: assuming we neglect gravity,the basickinematicfeatures of motion can always be referred to aunified system of time coordinates within anyarbitrarilychosen inertialreferencef rame. Accordingt ot his deflationary account of the situation, the point Russell was trying to make in 1914and 1922 was basically repeatingKant'sargument in the Metaphysical Foundations of NaturalScience,namelythat the relativity of kinematic perspectivesd oes not reallyc hallenget he rational ideal of an  Bergson'sb ook was released duringt he summer of 1922. It was republished the year after, augmented with several appendices( see Bergson 1999 and Bergson2009 for the English translation).  See also Russell 1925,5 6: the promotion of proper time suggests that we "abandon the old belief in one universal time".The same argument is repeated in ap iecef or the Encyclopaedia Britannica, "Philosophical Consequences of Relativity" (Russell 1926), whereR ussell explains that time, beingp rivatet oe ach body, does not constituteasingle cosmic order. all-embracing time serving as the backdrop of all determination of motion. In that sense, absolute time, just as absolute space, can be retainedasaregulative Idea stripped of anycosmic substance. How this should impact actual physics is, naturally, another matter. It maybeargued that Einstein'saccomplishment resided in showing that,when it comes to elucidatingt he spatio-temporal underpinningso ft he dynamics of moving bodies, the regulative Idea of absolutet ime is useless at best-as useless, that is, as the aether concept.Asmanyofhis colleagues, he believed thatcommon sense and philosophical understanding alike had to reform themselvest oe mbrace the new outlook-otherwise,t hey would be mere impediments to scientific progress.
If one is reluctant to dismiss Russell'soriginal appraisal of Relativity theory as one more expression of some deep-seated philosophical prejudice in favorof absolutes, then af ew hypotheses suggest themselves. WasR ussell on to something more substantial, in the spirit of his earlyrebuttal of relational conceptions of time and space, leading to the epistemological vindication of absolutem otion?³ Wash ee choing,r ather,W hitehead'sv iew that time, considered as the form of actual process, extends in some sense "beyond the spatio-temporal continuum of nature" (Whitehead 1925,1 81)? Or was he merelya pplying ab asic principle of philosophical prudence regarding such fundamental conceptsa s time, space or causality?
Whatever the answer mayb e, the example of Russell'sL owell lectures should encourageu st oa dopt,i nt urn, ap rinciple of hermeneuticr elativity (or symmetry) when it comes to reassessing certain episodes in the earlyphilosophical reception of Relativity theory.Bergson'se ngagementw ith Einstein is ac ase in point.M ore generally, Russell'sc autionary tale should prompt us to think carefullya bout the reasons that can bring well-informed philosophers to advocate an ideal of temporalu nity thati sprima facie at odds with the mainstream interpretation of physical theory offered by people in the trade.

2O nt he Formal Character of "Time"
My contention in what follows is thatthe difficulty can be traced backtothe formal nature of the time concept,ortobemore accurate, the fact thattime is best characterized in terms of form. The implications of this claim need to be clarified before undertaking anys erious research on topics related to the "philosophyof time".T he underlying intuition, negativelyf ormulated, is that what we call "time" can neither be athing-concept in the usual sense, nor aproxy for an abstract,r elational structure holding between pre-existing things. Time does not stand for an object or an empirical state of affairs, even one endowed with aremarkablyh ighd egreeo fg enerality.I nt his regard, it is very much like information, energy or matter-which, as Lenin remindsu s, is not ac oncept but ac ategory.A long similar lines, Wittgenstein argues that names such as thing, object, event, existence,o ri ndeed concept reallys tand for what the Tractatus,4 .126 ff, introduces as "formal concepts" (Wittgenstein 1974,3 3-34). That there are objects and events is not af act,i ti saconstitutive part of our form of representation. Time tooc annot be treated as anyotherelementoff act.And yeti ts formal character does not prevent it from having genuine content,⁴ conferringi ne ffect rational coherence on ap lurality of contrasting dimensionsa nd aspects of becoming. Accordingly, if time indeeddoes something for us, it cannot be reduced to amere intellectual device superimposed upon the varieties of temporalexperience (for instance, an orderingscheme for events happening "in" time according to relations of succession and simultaneity); nor can it be assimilated to an empty framework for the manipulation of metric variables.
The most perceptive among philosophersh avea cknowledgedt his special status of time in some way, even whent heir instinct led them to discard the usual conception of form as overlyabstract.Whether anyofthis should concern the physicist is of course debatable. Bergson and Russell had different views on this particular issue, but they did not stand very far apart regardingt he special philosophical status for the concept of time. At anyrate they both believed that once everything has been said about the physicist'sh andlingo ft ime measurements and the psychologist'se lucidation of temporal experience,t here is still room for ap hilosophical inquiry about the meaningo f" time". Now what are the indications that time indeedassumesthe status of aform, for lack of abetter word? The truth is that this theme runs throughout the entire history of philosophy. In the Physics,A ristotle emphasized the fact that time is not itself avariety of motion, thatitiseverywherethe same and cannot possibly flow at af aster or slower rate. In the section of his first Critique devoted to the Analogies of Experience,K ant famouslyi nsistedthat time itself does not change and cannotb ep erceiveda ss uch. Wittgenstein, in as ection of the Tractatus devoted to the formal nature of so-called "laws" in logic and physics (6.3611), claimed that therei s" no such thing" as "the passageo ft ime" (Wittgenstein 1974,83), which of course isn'tthe same as saying that time does not pass-whatever that could mean. Heidegger, who confessed to being interested in time and temporale xperience onlyi ns of ar as they could contributet oane lucidation of the question of being,a lso insistedi nSein und Zeit that from an ontic point of view,the most conspicuous aspect of an ontological approach to time was its formality (Formalität), af ormality verging on "emptiness" (Heidegger 1996,2 30). Arthur Prior argued on metaphysical ground that the present cannot be relativized withoutcompromising the very meaning of existence, thus drawingour attention to apoint of conceptual grammar: existencea nd coexistence are related in aw ay that is independentf rom frame-relative ascriptions of simultaneity (Prior 1970). Russell, as we have just seen, did not recoil from maintaining an "all-embracing" time form even though the new physics held the one even-flowing stream of time as the "relic of abygone age",toparaphrase his famous statement about causal laws. Bergson in turn, while conferringspecial status on lived, concrete duration, identified its generic form as am ultiplicity "resembling no other":asui generis qualitative multiplicity,atonce continuous and heterogeneous, incorporatingadouble principle of conservation and differentiation.⁵ One wonders what is to be gained from characterizings uch ac oncept as psychological time.
The list could go on. Each of these examples deserves to be carefullyspelled out: such at ask is beyond the scope of this contribution. Foro ur purpose it is enough to observethat,taken together,all of the aboveclaims exhibit acommon thread. They convergeinthe sense that time is not an object,nor asortalconcept applying to whatever particular instance we take to exhibit temporal features. Forl ack of ab etter word, time is af orm.
Givent he prevalence of this formal theme,i tw as onlyn atural for philosophers to approach Relativity theory with some circumspection. The shared feeling was that the significance of Einstein'sn ew insights into the nature of time could not merelyc onsist in proclaimingt he relativity and plurality of times, as if some unfortunate accident had struck the temporala ether and disrupted its  This conservation principle should not be confused with the principle of permanenceformulated in Kant's FirstA nalogy of Experience under the category of substance. If duration is deemed substantial by Bergson, it is in virtue of the dynamic continuation of the past into the present,ap rocess which clearlyi nvolvesm oret han either endurance or perdurance, while remainingf undamentally neutral with respect to A-time and B-time interpretations of time's "passage".A sf or the principle of differentiation, it is merelya nother aspect of continuation: the continuous weight exertedb yt he past upon the present implies that no moment of time can be repeated identically. Thus,B ergson suggests at emporal counterpart of the Leibnizian principle of indiscernibles that circumvents the concepts of substance and essence.
Time as Form: Lessons from the Bergson-Einstein Dispute inherent unity,leaving us with amultiplicity of dispersed temporalshreds.Here we mayt akeo ur cue from Gaston Bachelard: "when Einstein'sR elativity came along",h ew rites, "it deformed primordial concepts thatw et hought were fixed forever.From then on, reason multiplied its objections, dissociating fundamental ideas and then making new connections between them, trying out the boldest of abstractions" (Bachelard 2002,19). The implication, as fara st ime is concerned, is that the philosophical transformationb roughta bout by the new physics did not primarilyc oncern an enigmatic temporal substance that erring philosophers had previouslydefined in absolute, metaphysical terms.Dissociating fundamental ideas, trying new connections:the amount of conceptualization and problematization required to fit "time" into the relativisticf ramework suggests that something more is at stake thant he overthrowingofadubioust heoretical entity of the aether kind.F or the same reason, the fact that relativistic time can be givens traightforward operational meaning under certain usage, lending itself to consistent measurement,i sn ot enough to turn "time" itself into an empirical concept.The first step towards acknowledging the formal character of time consists in realizingt hatt he dimensions of experience that "time" is intended to capture are not necessarilyo ft he kind one maym easure (like a flow rate), much less count and sort out (like apples in abasket). As will become apparent,t his has little to do with the fact that time comprises as ubjective or psychological element.
How is time not an empirical concept,given that we measure it?Hereagain, we can onlyo ffer cursory remarks.T he following will suffice. Granted, we do measure durations in relation with particular processes. But the problem of time, properlys peaking,o nlya risesw hen it comes to coordinating such durations with av iew to the totality of durations within the universe. At that level, "time" must be treated as af orm effecting,i nR ussell'sw ords, the correlation of local times. As we shall argue, it is inseparablefrom an extended sense of coexistence. And yetp hysicists readilys peaka si ft herew erea sm anyd istinct "times" as therea re reference systems in relative motion, or ways to causally connect time-likeseparated events, thereby suggesting that something more substantial is at stake than the sheer multiplicity of temporalm easurements,a si f the object previouslyknown as "time" had been somehow pulverized. Such formulations are ambiguous at best.The onlyw ay to make sense of them is to include them in ac omprehensive account of time form in which measurement is but one dimension among others.
The same logic of object-orienteddiscourse bringsusto view relativistic time as at ime stripped out of some of its classicalf eatures:unity,uniformity,distant simultaneity or aconstitutive reference to the present moment.Thus, we customize the concept of time as if these wereo ptional elements in the package,e le-ments that one could assemble and re-assemble withoutc ompromising the integrity of temporalf orm. The bifurcation of temporalc oncepts into objective (physical) and subjective (psychological) sub-genres obeys as imilar pattern: it reinforcest he impression that time constitutes ap articularf ield of studyt hat one mayc hoose to approach from different perspectives, laying emphasis on this or thatp articular set of aspectse xhibited by temporalp henomena, pitting time consciousness against so-called "clock time",a nd so on.
Accordingtothe boldest among physicists,time mayturn out not to exist at all, as if time was again athing,the existenceornon-existenceofwhich could be in question. No amount of relationist medicinew illr id us from such category mistakes.D efining time as ar elationals tructure does not make it anyl ess real,u nless one endorses strong metaphysical views regardingt he natureo f emergence and the ontology of relations. Admittedly, more often than not the alleged "disappearance of time" is merelyaroundabout wayofsaying that,atthe fundamental level, the world is best described in terms of an atemporal theory, or perhaps that the physical world as described by our best scientifict heories does not exhibit af ixed temporal backdrop, au niversal arena of change. This point has been made in different ways,a nd on different grounds,b ys uch authors as LeeS molin, Carlo Rovelli, Julian Barbour.One mayf or example underscoret he fact that the Wheeler-De Witt equation, sometimes described as the wave-function of the universe-under the disputable assumption that the universe as awhole behavesasaHamiltonian system-,does not include anyreference to an external time.
There is much philosophicalconfusion behind the idea of atemporal dynamics, but the theme strikesasympathetic chordwith the formallymindedphilosopher of time because the fact that dynamics can be expressed without time is consistent with the sense that time itself does not change, and accordingly does not exhibit dynamic features.When pressed further,h owever,t he natural philosopher that lays dormant in every physicist is tempted to utter something like this: "Ih avel ooked for 'time' everywhere, both at the microscopic and cosmological levels, and Ihavefound nothing…".This is baffling,for what on Earth did one expect to find?T here is something vaguelyr eminiscent here of Yuri Gagarin'sfamous pronouncement on returningfromhis orbital trip aboard Vostok 1: "Isee no God up here!".Ifthe inexistenceoftime is aprovocative wayofsaying that the universe is not bathing in atemporal aether of sorts, the claim is perfectly acceptable, albeit misleading. It onlyconfirms the fact that time is not itself an object or process, not even ah ighlyt heory-ladeno ne, such as the expansion of the universe described by current cosmology.
Once we relinquish the notion of time as acontainer of change, we maystill want to ask what it means for thingst ob ein time in the first place. Aristotle's Physics raised the question onlytowarn us about the limits of anyanalogywith the fact of occupying aplace. Yetinthe same book, time is sometimes likened to an envelope of motion, an imagew hich, to be properlyu nderstood, would require rising to ah igher degree of abstraction. Curiouslye nough, problems of this naturea re almostn ever addressed in current debates over the substantival or relational nature of time and space-time, whose main focus is on knowing whether ap articularo bject or structure, defined in geometrical terms,e xists in its own right,whether it can be grounded in more primitive elements, and the like.
To be fair,the tendencytoreify time and treat it as athing-concept is largely counterbalanced by the operationalist proclivity to frame all temporali ssues in terms of what we can actuallybring the concept of time to do for us: for example, correlating measurements of durations. From this perspective,wem ay want to define time as the quintessenceo fa ll time-keepingdevices.A tt his level of generality,time appears as an ingenious labellingprocedured evised by the human mind in the course of its evolution. Scientists inherit from this device; they have onlymanaged to give it alevel of mathematical sophistication thatenables them to sort the variable configurations thatconstitutethe history of the universe and build everythingf rom there. Yet, at the end of the day, such deflationary accounts of time leave everything untouched; they raise the same issues as the bolderm etaphysical views about the "disappearance of time".O ftentimes, the more empiricallym inded philosophers will offer sweeping ontological pronouncements to the effect that time, once again, doesn'te xist,n ot in virtue of some substantial theory about mathematical constructs,b ut simplyb ecause a universal time-keepingd evice evolvedb yh igher organisms to make sense of their environment is fundamentallyn od ifferent from anyo ther human artifact. It is easy to see how an agreement can be found at this basic level with philosophersa ttachedt ot he idea of time as as ubjective form of experience:apragmatic, historicized reformulation of the ap riori will do the trick. But it is only as mall step from this to the claim that time is nothing out there,o rt hat its very passagei sb ut an elaborate cognitivei llusion. And more often than not, such considerations secretlyt rade on ah ypostasized representation of time as some fundamental process underlying all processes. Is this process occurring in the mind only, or does it have genuine objective, physical grounding? If one is not in the mood for metaphysics,a ne asy wayo ff udging the problem is to refer to John Archibald Wheeler'sm emorable dictumt hat time is the easiest waynaturehas found to keep everythingfrom happeningatonce. We can do better thant hat.

3AFunctional Approach to Form
These scattered and sketchyremarks all point to the same direction: to assess the philosophical relevance of the physicist'spronouncements about time, it is welladvised to approach them in what Carnap described as the formal mode of speech, not onlybecause temporalconceptsdonot necessarilyhavedirect intuitive,empirical or material content,but morefundamentallybecause they generallyoperate at higher level of abstraction than anyclassifying concept or category.Their function, Is urmise, is to provide ac oherent framework for ac luster of related issues pertainingtobeing and event,identityand change, structure and process, purpose and causality,etc. It is to address such concerns that Kant came up with adoctrine of the "order of time" in his Analogies of Experience,bringing together the categories of substance, cause and community,toachieveaconsistent and unified account of permanence,s uccession and simultaneity.
From this standpoint,itisclear that 'time' cannot be amere placeholder for whatever physical theory deems relevant to the mathematical analysis of becoming.Aphilosophical account of time must somehow resonate with the entire cluster of problems mentioned above, includingthosestemmingfrom the implicit reference of temporalpredication to apresent moment ("now", "then"), which mayormay not be construed as the mark of an irreducibly subjective standpoint. By remindingu so ft his simple fact,t he philosopher is not claiming privileged access to as pecial object that would lie outside the reach of scientific understanding.H ei sm aking ap oint about the kind of expectations thatc ome with the concept of time. Such expectations and anticipations, as Bergson oftene mphasized, implythat we do not assumefrom the outset an unbridgeable gapbetween the experiential aspectso ft ime disclosed in livedd uration and the rules governing our use of the parameter t in physical theory.Otherwise, whycontinue to use the samew ord( " time")f or both? Fromt hats tandpoint,e quating time with am athematical object effecting the correlation of time measurements doesn'tdoitm ore justicethan holding it as the immutable and irreduciblysubjective form of inner sense. Our framing of time concepts needs to be checked against the completet heoretical background that motivates our reflection on the nature of time in the first place. That is whyn either the mathematical nor the transcendental understandingo ff orm can exhaust the meaning of time form. In fact,e lucidatingt he formal character of time mayw ell requireathorough examination of the entire spectrum of temporalexperience. In the process, time mayt urn out to be av ery peculiark ind of form indeed, af orm of the non-Aristotelian and non-Kantian variety-aform resembling no other,toparaphrase Bergson, af orm thati si ns ome waya dherent to its content.
The crucial question, in anyc ase, is the following:w hat does it mean to work on ac oncept,r ather than put it to service?G eorgesC anguilhem nicely puts it in at ext about Bachelard: To work on ac oncept is to vary its extension and comprehension, to generalize it through the incorporation of exceptional traits,toexport it beyond its region of origin, to take it as a model or inversely, to searchf or am odel for it-in short,t op rogressively confer upon it, through regulated transformations, the function of af orm.⁶ Forour purpose, assuming such afunctional stance, makinguse of form as a regulative idea, appears more productive than attemptingt of lesh out its meaning and content from the outset in ad efinition. Admittedly, philosophers have generallys hown more interest in "varying[ time's] extension and comprehension",than in "generalizing it through incorporation of exceptional traits".Physicists on the other hand, more particularlythose involved in the development of Relativity theory,haveachievedanunprecedented level of generalization of temporal concepts by showing that as ag eneral rule-ar ule which onlyb ecomes conspicuous in certain special conditions or limiting cases (when dealing with velocities close to the speed of light,f or instance)-,t emporala nd spatial aspects must be handled together as part of one single mathematical form in which they appeartightlywoven, rather than merelyjuxtaposed. The elucidation of the structure of relativistic space-time certainlyconstitutes an importantlandmark in that respect.Onone level, it offers aparadigm of the formal approach to temporalissues. It is also quite helpful in dispelling certain misconceptions such as the alleged "slowing down" or "dilation" of time.⁷ Yetitremains to be seen in what sense time itself assumes the function of aform once it has been mergedin this overarchings tructure.
Remarkablye nough, the generalization achievedb yt he space-time approach mayi nfact amount to a specification of time form,and arguably to ar eduction of its original scope, as indicated by the narrowing down of absolutesimultaneity to sheer facts of coincidencea nd the subsequent promotion of local time, i. e., aquantity measured along spatio-temporal worldlines.Deprivedfrom the independence it enjoyed in the classicals etting,w heref our-dimensional  Canguilhem, "Dialectique et philosophie du non chez Bachelard" (1963), quoted in Hallward and Peden2012,13. This quoteisfeatured as an epigraph of each of the volumes of the "Cahiers pour l'analyse" published between 1966 and 1969.  Amorecompellingimageisthat of space-time itself (its metrical features) actingassome sort of latticeorfilteringdevice, forcingthe flow of time to fork out and take spatio-temporal detours that turn out to be temporal shortcuts (i. e., routes of lesser elapsed duration). space-time was merelythe Cartesian product of temporaland spatial dimensions with no unifiedmetrics, "time" has clearlynot disappeared. It survivesindifferent guises, deprivedo fs ome of its familiar privileges. But is it real time? Rather than brushing the question aside as an expression of philosophical conservatism, Is uggest we rephrase it in formal mode so thatt he search for real time serves as ac atalyst for the elucidation of time form,i nsteado fm irrorings ome pre-existings tandard-be it intuitiveo rc onceptual-of what should count as the primordial meaning of "time".
The samecircumspection is in order when dealing with what Bergson holds as the main property attached to real time in physics:i ts unity or universality. Evidently, acknowledging ap lurality of time forms would defeat the very purpose of adopting af ormal stance in the first place. If we are serious about form, therec an onlyb eo ne time form. The challengei st oe xplain how such a form can accommodate ap lurality of time measurements.

Real Time is Measured Time!
In that respect,Bergson comes across as somewhat morep rudent than manyof his colleagues, includingthe earlyRussell. Forone thing,while advocating asingle universal time, he left the question open as to the appropriate theoretical format that could instantiatet his metaphysical claim at ap hysical level. In particular,h en ever entertained the notion that it would be philosophicallys ound to redeem Newton'sabsolute time, or to maintain it in relativized form,inthe manner of Poincaréo rL orentz, by granting privileged status to conventionallychosen referencef rames bearing true time. This would have run against the general orientation of his discussion of the "cinematographic mechanism of thought". Absolute, uniform time, likea ll concepts of time modelled after the mathematical time-dimension manipulated by classicalm echanics, whether in parameter or coordinate format,i mplies preciselythe kind of overall framing and schematizing of real changet hat is exposed and criticized in CreativeE volution.
In every case, the reconstruction of actual experience effected by cinematographic intelligenceimplies referringparticularprocesses to the abstraction of a "single representation of becoming in general […], abecomingalways and everywheret he same, invariablyc olourless" (Bergson 1998, 304). Bergson was naturallys uspicious of the metaphor of universal time flow,w hich in effect treats time as an all-embracing medium of changeu nderlying every particular duration. It is worth noting that it matters little at this point whether time is one or several, whether absolutet ime is meanti nt he original sense intended by Newton, or in the relativized sense underlying the use of aunifiedsystem of tem-poral coordinates within each particularr eference frame. Thel atter solution merelym ultiplies and aggravates the problem by conjuring up the monstruous imageofa"hyper-cinematograph" of sorts, projectingasmanyglobal renderings of actual becominga st herea re ways of framing it accordingt op articulark inematic perspectives.⁸ While it maystill be appropriate for physical purpose, Bergson for one did not see anyphilosophical benefitinsalvaging such aconception, let alone giving it genuine ontologicals tatus.
Besides, the knee-jerk reaction of dismaytriggered by anymention of universal time in relation to Relativity theory should not overshadow this obvious fact: if the philosopher'sh iddena genda was to vindicate the conceptual framework provided by Newtonian time (the so-called universalt ime symbolized by the Greenwich meridian clock), he would have chosen arather curious route to achievethis -first establishingthe inherent limitations of all aether-based versions of Relativity (chapter Io fDuration and Simultaneity), then systematicallyc ontrasting real time with the relative and ultimatelyf ictitious nature of all framedependent determination of time. Likewise, if all he had in mind wast or escue absolutetime, the paradoxical claim to the effect that Relativity theory bringsout the "unity of real time" even more clearlyt han classical mechanics would remain utterlyi ncomprehensible.
In view of all this, the notion that Bergson is clingingtoanobsolete conception of absolute time for purelyp hilosophical reasons is simplyp reposterous. The heart of the matter lies in what the critique of the cinematographic illusion brought to the fore, namely the framing function attributedtotime in both relativistic and non-relativistic setting.P rima facie, the search for real time findsi ts motivation in ar eaction against framedtime.B ut to elucidatei ts concept on its own terms, we need to contrast it with what Bergson describes as fictitious times, i. e., the mathematical expression of the anamorphict ransformations affecting temporalmeasurement as we shift from one reference frame to another.Granted, relativisticeffects such as length contraction and time dilation are commonlyobserved. Such effects, however,donot make these times less fictious, for they can always be construed as perspectival artefacts resulting from the use of arbitrary frames in the account of elapsed durations at ad istance. The concept of real time,onthe other hand, is meant to reflect certain aspectsoftime that are independent from anys uch framing,yet no less measurable for that.
The point deservese mphasis:t he critical distinction between real and fictitious time operates within the very domainofmeasured time. Contrarytowhat is generallybelieved, real time is not another name for pure duration. Duration and Simultaneity unequivocallyi ntroduces it as av ariety of physical time. It is the time actuallym easured (or potentiallym easurable) by ar eal clock attachedt o ap ortion of matter.A ccordingly,when Bergson insistst hat real time is not the kind of thing that can be torn apart and dismantledbythe mere effect of relative speed, he is reallymaking apoint about the grammar of physical time, which he argues cannot be handled as freelyasamathematical variable. This is af ar cry from merelyp laying subjective or livedt ime against physical or measured time, even if all determinations of real time ultimatelyl ead backt ot he conditionsi n which real observers perform actual measurements.
To repeat, real time is essentiallym easured time.I ti st he time of matter, to the extent that matter lends itself to measurement.F or this reason, when it comes to appreciating the motivations behind Bergson'se ngagement with Einstein, it is entirelym isleading to portrayh im as an advocate of the primacy of livedo rp sychological duration. If the philosopher and the scientist werec onfrontinge ach other from the two opposite sides of the subjective/objective divide, they would be speakinga tc ross-purpose and their quarrel would appear pointless,t urning around ah omonymous use of "time".A ccordingly, Einstein would be justified in proclaiming that there is in fact no third time-no "philosopher'st ime" besides the time of physicistsa nd the time of psychologists.⁹ For once we have accounted for the metric properties of time and for the qualitative features attachedt of elt time, it seems there is indeed nothing left to study.¹⁰ That,h owever,i sn ot reallyt he issue. The formal understanding of "time" is the keyhere: if there is no such thing as the "philosopher'stime",nothird object requiringspecial scrutiny, it is onlyowing to the fact that time is not an object in the first place. So, we mays ay thatE instein was right after all, although not in the sense he himself intended.

5T he Prospect of Universal Time
In anutshell,Bergson'sclaim is not that real time is lived, but that it is livedand counted, livedand measured. It is livedevenmoresoasitiscounted and meas- This blunt statement can be found in the transcripts of the brief exchange that followed Bergson'slectureduringEinstein's1922visit at the CollègedeFranceinParis (Bergson1999,158). See During2 020, 44-45.  It should be noted that Einstein himself readily acknowledgesthat apsychological or intuitive apprehension of time is necessarilypresupposed by the actual use of measuringinstruments (i. e., the readingo fc locks). This entails no conflict or contradiction, as long as we agree on a correspondence scheme linkingp erceptual observations and theoretical constructs.
Time as Form: Lessons from the Bergson-Einstein Dispute ured. Morei mportantly,h owever,i ti su nique. Thereinl ies its most conspicuous characteristic.
The underlying metaphysical view can in turn be expressed in bothmaterial and formal mode. In material mode, Bergson is defending the view thatthe universe as aw hole endures: as such, it is fundamentallya nalogous to lived, conscious duration. The deeper motivations behinds uch av iew need not concern us at this point.S uffice it to saythat the idea of acosmic temporalwaves weeping across the entireuniverse is stronglysuggested by common sense,not exactly through analogical reasoning,but by virtue of ap rinciple of similaritya llowing for gradual extensions from local to global. In anyc ase, for Bergson universal time ultimatelyr emains ac onjectureo rh ypothesis thatm ust be appraised on its own philosophical merits rather than as ab lueprint for an alternative physical theory of Relativity along Lorentzian or Poincarean lines.¹¹ In formal mode, the reaffirmation of the "unity of real time" stems from a profound discomfort with the ontological slacknessresulting from the metaphorical spatialization and reification of time. It is one thing to say, for example, that there are as manytime-systems as thereare reference frames in relative motion, or as manye lapsed durations between two time-like separated events as there are ways of connectingthem causally; it is quite another to take the multiplicity of temporalmeasurements associatedwith particular movementsorprocesses as evidence for an actual multiplicity of a-synchronous durations unfoldingi n space, as if these were themselvesp rocesses of some kind. At af undamental level, the search for real time is an attempt to rectify the misconceptions fostered by the overused metaphor of time'sf low.
The remedy, once again,i st or eaffirm the essential unity of time forma sa matter of principle. However,this cannotbeachieved entirely apriori. In keeping with the general orientation of Bergson'se mpiricist method, the onlyw ay to effectively recover that sense of unity is to examine the actual operations carried out by the physicist,r ather than dismissing measured time indiscriminately or simplypositing genuine duration and the ideal of subjective unity as atranscendental pre-condition for all temporal determination. But this raises in turn acrit- ÉdouardGuillaume, an earlytranslator of Einstein in French and editor of Poincaré'sscientific writings,entertained just such aprospect.His theories arementioned in Duration and Simultaneity with some reservations (see Bergson 2009,133;302 -302).A nother case in point is Herbert Dingle.Anobstinateopponent of Relativity theory,healso authored along introductiont o the first English translation of Durée et Simultanéité,claimingthat Bergson,objecting to the idea of asymmetrica gingi nt he standarde xposition of Langevin'st win paradox, had thereby advanced "ap erfectlyr elevant argument even from the physical point of view" (Bergson 1965, xvii). Fortunately, this introduction was not included in further editions (Bergson 1999). ical question. If real time lends itself to measurement,ifthe structure of physical theory implies away of coordinating the results of time measurements,tow hat particulara spect of physical time does real time correspond?
The difficulty with Einstein'sR elativity is that "time" appears to be all over the place, refracted at different levels within the entire theory.O no ne level, space-time itself can be said to assume some of the traditionalf unctions of time form. As we have seen, the phantasmali mageo ff rozen becomingt hat is conjured whenever space-time is considered as ag eometric object (or block)l iterallyl aid out in four dimensions, conceals as ophisticated machinery that in fact operates like ah yper-cinematograph, offering infinitely manyp rojections of becoming-as many as there are reference frames. All these projections are virtuallye mbeddedi ns pace-time and directlyr ecoverable from its metrical form. The Lorentz equations express in algebraic terms the wayt hese projections can be coordinated through appropriate transformations. To the extent that space-time thus achieves af ormal totalization of becoming, it suggestsi tself as as ubstitute for absolute time, but it can onlyd os oa tal evel of generality that does not even begin to address Bergson'sc oncerns. The immutable unity of spatio-temporal forms ymbolized by the Lorentzian metric signaturet urns out to be toolarge to conveythe temporalunity of interlockingdurations within the actual universe. The space-time of Special Relativity has onlytangential relevancetothe variablycurved space-time of General Relativity:itis, in the end, an ideal mathematical object.A ss uch, Bergson believes it does not have any straightforward lesson to deliverr egarding the nature of time-af orm adherent to real becoming. That is the main thread running through the last chapter of Duration and Simultaneity devoted to four-dimensionals pace-time.
At another level, we find coordinate times attached to particularr eference frames (or equivalencec lasses of coordinate systems), as well as proper times measured along individual worldlines. Obviously, these two determinations of time do not merelyc oexist alongsidee ach other;t hey are closelye ntwined in the metric of space-time. Yet, despite this deep mathematical connection, it seems as if "time" had been splita part and projected upon different planes of expression as ar esult of its entanglement with space. In actual use, the relativistic framework displays ac onstant interferenceo fp arameter-time with coordinate-time, but the waythis oscillation between local and global time is reflected on ad iscursive level reinforcest he feeling thatw ea re dealing with heterogeneous aspects of temporalf orm. Meanwhile, clocksa re moved around, synchronized (either locally, or at ad istance, by exchangingl ight signals) and desynchronized (owing to relative motion, and more importantly, dynamic factors). They time events and measure durationsw hile mutuallys urveying each other in some sense, notwithstanding the disruptions. Andi fi ti st rue that time itself is never directlym easured, if it is better defined, following Carlo Rovelli'ssuggestion, as an "exchangerate" between othermagnitudes endowed with more immediate physical content,then the perfect clock is ultimatelynothing but the universe as awhole (or alternately, the most comprehensive theory of that universe). This is yetanother confirmation that the form of time cannot easily be pinned down, leaving open the question of wheret ol ocate its unity and universality beyond the form of space-time itself.

6T he Lure of LocalT ime
As far as the basicprinciples of physical theory,Bergson views himself as athorough relativist.Hehas givenupabsolutespace and its material counterpart,the aether.Relying on aprivileged frame is not an option, especiallyift his involves redeemingabsolutet ime in the classic form of frame-time.Clearly, the "unity of real time" must lie elsewhere. Can it be found in propert ime,the local time introduced by Einstein in his analysis of the logic of measurement,based on rods and clocks? Herei satime marked out by the actual strokes of ac lock,atime registered on the spot,s ot os peak, wheret he actiont akes place.¹² It seems time could hardlyb ea ny more "real" than that.Y et,g iven Bergson'sc riticism of the philosophical abuse of mathematical, homogeneous time, local time could onlya ppear to him as af urther development,r ather than an overthrow, of the abstract representation of time epitomized by Newtonian absolute time. This deep-seated conviction certainlycontributed to downplaying the real novelty behind the Einsteinian use of local time. Fort he most important lesson to be taken from Relativity,alesson which manyphilosophers and physicists alike did not always fullya ppreciate, is not so much the fact thatt ime is relative to the observer-that is, to the choice of an arbitrary frame of reference-,but more profoundlythat time is relative to the varyingintensities of motion affecting the observer in the general case where, being accelerated, it cannot be assigned asingle inertialframe. Thus, proper time is typicallyreferred to aworldline followed in space-time by aportion of matter (a clock, ahuman observer)undergoing various degrees of dynamic acceleration. It is, strictlyspeaking, al ength measured along such aworldline,alength whose mathematical expression happens to be  The definitions of proper time varyfromone textbook to another,dependingonthe emphasis one wishes to layonthe intrinsic (frame-invariant) aspects of the situation.Some refertothe time measured by clocks sharingthe same motion as the observer (i. e., clocks at rest in the reference frame of the observer), while others mention the time registered by aclock "carried" from one event to another. independent of anyp articular framing.T aken as the paradigm of local time, it encapsulates the following basic idea: relativistic time is essentiallyapath-dependent-rather than frame-dependent-magnitude; it is relative to the observer to the extent that the observerisd ynamicallyrelatedtothe universe as awhole.
Now,owing to the metric of relativistic space-time, the shortest (i. e., geodesic) path in space-time happens to be the longest in time. Langevin'sfamous paradoxofthe twins offers adirect illustration of this general point; hence its paradigmatic function.¹³ In Bergson'se yes, however,p roper time is philosophically useless: the drawbacks of its path-dependency far outweigh the philosophical benefitso fi ts frame-invariance. Itsp ivotal role in Relativity theory appears to him as af urther stepi nt he direction indicated by Descartes:that of at horough geometrization of matter and motion. Morei mportantly,t he suspicion is that proper times measured along stretches of space-time do not have anything distinctly temporal about them besides the fact that,being predicatedupon continuous motion in space, they suggest an atural temporal orderingofe vents along "time-like" paths-aproto-temporal schema that Piaget would describea smere "spatial succession" (Piaget 1969, 26). In other words, what proper time has to offer is at best alocal expression of causal order.But as amonotonouslyincreasing parameter defined along space-time curves, it is reallya4D spatial magnitude in temporal clothing.T ot he extent that it captures something of the flow of causality propagating from place to place, it contributes to the spatio-temporalrepresentation of becoming, but the onlyway it can infuse agenuine sense of temporalunfolding is by relying on apre-existing intuition of duration, the prototype of which, Bergson argues, ultimatelyb ringsb ack consciousness in the form of livedd uration (i. e., asuccession of events without anyclear-cut distinction between past and present states,conjoined with the perceivedsimultaneity of multiple flows distributed across space).
Thus, when some philosophers followed Langevin'ss uggestion thatBergsonian real time could be identified with the physicist'sp roper time, and thereby restore some sense of invariance and unity beneath the relative projections of frame-time,¹⁴ they could not be fartherfrom Bergson'soriginal intent.For strictly  The twin paradoxepitomizes what Taylor and Wheeler describe as achronogeometrical principle of "maximalaging".Inunformal mode: "The worldline of afreestone has maximum wristwatch time between adjacent events" (Taylor,W heeler and Bertschinger2 006,s ection 1.6). "Wristwatch time" is another name for proper time; "free stone" stands for anys ystem in uniform motion. According to the authors,the principle of maximal aginginrelativistic spacetime is structurallya nalogous to Newton'sf irst lawo fm otion in classical spacetime.  "The philosophera dopts the perspective of proper time,the time particular to each [observer]. The physicist adopts the perspective of acommon time: the questions he raises bringhim to Time as Form: Lessons from the Bergson-Einstein Dispute speaking, proper time, though non-perspectival and invariant,i sn othing but a local magnitude measured along spatio-temporal paths. When such paths are combined with moving clocks, it seems we have somehow captured time and fixed it along its course, so to speak. Indeed, each clock can be said to give an accurate measure of its ownp roper time as it moves around in space. But it is importantt or ealize how unfamiliar this variety of "clock time" reallyi s. Forone thing,proper time does not come equipped with anysense of simultaneity besides trivial facts of local coincidence( i. e., intersections of space-time paths).R egisteringt he time of the action onlyw herei tt akes place, proper time entirelyl acks the kind of "thickness" or perspectival depth attachedt o the idea of real, extensive becoming, to the point that one can doubt whether it has anyi mmediate temporal meaning.¹⁵ The fact is that locality without perspective is not enough. To give proper time its full temporals cope, we need to associate it from the outset with some form of globaltime, or at least asynchronizing procedurea llowing relations of simultaneity to occur at some level. Otherwise, it is at best afibre-time of sorts: more elastic in some respects, yetinthe end no less homogeneous and abstract thanthe more familiar frame-time underlyingt he use of time coordinates.Onlyw hen temporal fibres align and co-moving local observers can be said to share areference frame, do we recover asense of global time, albeit ar elativized one. When all is said and done, Langevin's universal use of proper time does no more thano ffer am athematical substitute for the classic figure of time within an ew chronogeometricalf ramework. The question of the roots of time'su nity remains open. ForB ergson, it cannot be properlya ddressed on strictlyl ocal grounds.

7C oexistencei nT ime: the Real Issue
The inherent limitation of purelylocal definitions of time is indicative of the extent to which our expectations regarding time formare dependent upon the more basic intuition of coexistence in time. From Plato's Timaeus through Aristotle's Physics,d own to the "all-embracing" universal time of modern mechanics and Kant's Third Analogy of Experience,p hilosophersh avee ntertained the idea that time is an envelope or sheaf of becoming, and henceamedium of coexistence, even before it can be defined as ameasure (or "number")ofl ocal or coscompare the proper times of different observers" (Langevin, "Le temps,l ' espacee tl ac ausalité dans la physique moderne",alectureatthe SociétéF rançaise de Philosophie, October 19,1911, quoted in Bergson 2009,3 82).  CordF riebe makes as imilar point.S ee Friebe (2012). mic motion.¹⁶ Generalizing from this idea, we reach the conclusiont hat time's primary functioni so ne of co-ordination-ac onclusion corroborated by Piaget's research regardingt he development of temporal frameworks in children. More precisely, time form is what enables us to make sense of aplurality of durations unfolding together,n ot onlys patially, by virtue of being part of the same universe, but temporally, by virtue of being together in time. Time is what bringstogether durations conceivedascontemporaneous, if not simultaneous in the strict sense.
Bergson inherits from this rich tradition. Like others before him, he advocates the formal unity of time as ad imension of both changea nd coexistence. But the concept of duration changes the deal by severing the form of time from the extensive scheme of number typified in the parametricaluse of proper time. Accordingly, justasproper time takes on genuine temporalmeaning when it is grounded in livedd uration-otherwise, whyi nterpret it as al ength of "time"?-,t he operational definition of simultaneity at ad istance by wayo f light signals,a nd the subsequent foliation of space-time into frame-dependent planeso fs imultaneity,m ust ultimatelyb er eferred to what the third chapter of Duration and Simultaneity describes as the lived simultaneity of flows,r ather than instantaneous events. Taken in this broad sense, simultaneity escapes the narrow definition of instantaneous simultaneity at adistance which Einstein famouslys howed to be relative to the choice of ap articulars ystem of reference. Bergson'se mphasis on the "unity of real time",t ogether with his endorsement of "the one and universal time",t akes its full meaning in this perspective. What is at stake behind the issue of simultaneity is no less than the possibility of recovering am easure of connectedness and unity-am eaningful sense of community-in auniverse thatthe relativisticoverhaul of the concepts of simultaneity and duration have seemingly "disfigured" (Bergson 1946,3 01-303). The intuition is that the consistent reappraisal of simultaneity as an inherent feature of time form must deliveraninsight into the cohesion of the temporal fabric at a cosmic level.
If that was indeed Bergson'sintent,itisfair to saythat he was not very successful in driving the point home. His famous Paris meetingwith Einstein (Bergson 1999,154-159) was hosted by the Société Française de Philosophie on April 6, 1922 in the margins of aseries of lectures givenbythe physicist at the Collège  In his 1770 Dissertation (section III, §14), Kant had alreadyi ntroduced simultaneity as the most important consequenceo ft ime, insistingo nt he necessity to acknowledge simultaneity as arelation in its own right,rather than as as horthand for the non-successive.Thus,s imultaneity is the expression of the actual coexistenceo ft hings joined in the same moment of interaction: as such, it manifests the ubiquity of time.
Time as Form: Lessons from the Bergson-Einstein Dispute de France. The exchangebetween twoofthe most brilliant minds of the time has often been described as the intellectual equivalent of the ultimate fight in a heavyweight wrestlingc hampionship.T he press naturallyg avei ts ubstantial coverageatthe time. Several chronicles and historical works have since provided informativeand somewhat entertainingaccounts of the circumstances surrounding it.¹⁷ But the editorial dramatization of this altogether disappointinge pisode has had several unwelcome consequences. It has led some critics and commentators-bolstered by Einstein himself, as it appears-to overemphasize certain peripheral issues at the expense of more fundamental ones.
Acasei np oint is the status of "absolutes imultaneity",defined as simultaneity at-a-place( i. e., local coincidence). During the Paris meeting,i nordert o triggeradiscussion with Einstein, Bergson had seen fit to present as ection of his upcomingbook on Relativity theory,i nwhich arather convoluted argument is made for an extended use of the concept of simultaneity beyond point-likeoccurrences. Thissomehow encouraged the false impression that he was questioning the physicist'sr eliance on facts of local coincidence( i. e., two events occurring simultaneouslya tagivenp oint in space), when his aim was merelyt o question the implicit assumptions underlying anyi dea of simultaneity.H ea ttempted to do so by showing the hold thatc ertain geometrical representations (such as the idealized point-likee vents) have on our conception of what counts as absolute, wittilyreferring to the fictional viewpoint of relativistic microbes (see During 2020,40-42), but the fact is thatthe more substantial underlying issues werebarelyaddressed in the pages he had chosen to read from. So much so that we are todayi nt he difficult position of having to provide ar ational reconstruction for an argument that the twothinkers could not actuallyhave. Forthe commentator,this impliesperformingsome sort of ventriloquism in Bergson'sname.
Let us give it atry,tothe risk of anachronism. The observation thatsimultaneity at adistance, being frame-relative,loses all objective meaning in relativistic setting,i sg enerallyb elieved to have far-reaching and devastating implications for the philosophicalu nderstanding of time. Some arguet hat it inevitablyleads to its fragmentation into akaleidoscopic multiplicity of temporal projections,e ach referencef rame bearing,s ot os peak,i ts own time.B ut is it trulyt he case? To make temporals ense of such relativization in the first place, isn'titnecessary to set it against the background provided by some notion of spatio-temporal coexistence? Bergson, for one, believes thatt he primitive meaning of simultaneity is foundedu pon the actual dynamics of interlocking "flows" of mattera sa pprehended by some perceptual event.H et herefore as-  See Paty (1979); Biezunski( 1987) and; Canales (2015).
sumes the notion to be richer and more concrete-if less global-than the one suggested by "all-encompassing" planes of simultaneity cutting across the entire universe.
Others consider the relativization of simultaneity and the ensuing disruption of time as evidence of the need to account for temporal becominginstrictlylocal terms-i. e., in terms of proper time in the sense defined above. But as suggested, this onlys eems to aggravatet he problem. As Mauro Dorato nicelyp uts it,o nce we have givenupthe notion of aworld-wide advance of nature, of a 'now' moving like afront-waveonthe ocean of becoming, if we nevertheless want to retain asense of the overall temporalunity of the cosmic process, "the water provided by an uncorrelated, non-denumerable set of narrow creeks,each of which, representingt he proper time of aw orldline, 'flowing' at ad ifferent rate, maya lso proveinsufficient" (Dorato 1995,184). While the multiple perspectival projections of framed time at least obeyed uniform transformation rules (the symmetries of the Lorentz group), the intrinsic (i. e., frame-invariant) approach to temporal becomingseems to leave us with an utterlypulverized time: amultiplicity of loosely connected threads of proper time with no coordinating principle besides the metric structure of space-time itself and its underlyingtopology. General Relativity pushes thingso ne step further,f orcingust oacknowledge that aglobal temporal framing is unavailable as am atter of principle. In variablyc urved spacetime, whereMinkowskispace-time onlyholds locally, thereisnostraightforward wayo fd efining planes of simultaneity:t he twin paradoxb ecomes the general rule (see below, section 8).
At this point,itwould seem as if we ought not be concerned with figurative models of time flow and resign ourselvesinstead to strippingthe concept of time from anyglobal scope. But if coexistenceisassumed as aconstitutive dimension of real time,that would be tantamount to denying the existence of time altogether.The very possibility of conceiving of beingsa nd events as enduring together hangso nthec oordinating function of time,b eyond the trivial mode of coexistencesuggested by the generic form of space-time itself (or its phantasmalcounterpart,t he 4D "block universe"). As mentioned before, the kind of unity achieved by space-time, whether we picture it as as olid made of agglomerated fibres,orasaporous and fluid medium, remains essentiallyabstract.Asaresult, the coexistenceitexhibits is trivial at best and has nothing specificallytemporal about it.Thingscoexist in the sense that they are part of the samespatio-temporalform. But what Bergson argues about absolutetime is true of space-time too: whether we form the imageo f" an immenses olid sheet" (Bergson 1946,220)o r of "an infinity of crystallized needles" (Bergson 1946,219), in both cases we are committingacategory mistake because the space of coexistence itself is in fact treated as athing laid out in space. If the representation of threads of becoming congealed in a "block universe" serves anypurpose, it is thatofemphasizing the need to come up with an on-trivial and more robust conception of temporalc oexistence. The challengeistoachieve this without collapsing coexistence on the usual figures of global simultaneity.
But,toreiterate, there is no reason whyphilosophical reflection should confine the meaning of distant simultaneity to the physicist'sconcept of world-wide instants( planes of simultaneity). Fixings imultaneity relations between spacelike separatede vents by means of appropriater eferencef rames( i. e., systems of coordinates), implementing this through electromagnetic signallingp rocedures, is but one wayt oc onstrue distant simultaneity.A nd the global temporal perspective obtained from the use of coordinate systems by no meanse xhausts the meaningo fc oexistence. As am atter of fact,t he space-time framework alreadye xhibits patterns of simultaneity thata re neither global nor strictly local. We mayr efert ot hem as instances of regional simultaneity.I nterestingly enough,t hey displayi ntrinsic (i. e., frame-independent) characters, in the sense that they can be directlyr ead off from the invariant topological structure of space-time underlying the causal order.

8T he TwinsI :R egional Simultaneity
The twins' story of separation and reunion, as introduced in 1911 by Langevin,¹⁸ is at ouchstone in this respect,b ecause it provides as traightforward, almost graphic stagingofthe oddlydisjointedcoexistence of two distant flows of duration unfoldingand dephasing in parallel-or in real time,asitis. Despite the disruptions and discrepancies affecting anyattempt at ac ontinuous assessment of standard simultaneity relations between the stay-at-home and the traveller,their mutualh istory irresistiblyc onjures the imageo fasheaf or envelope of shared time. One cannot simplyi gnore this on account of the irrecoverable character of absolute simultaneity,a sc ommonlyu nderstood by the physicist.M yconten- Paul Langevin's1911 exposition does not mention "twins" but atravelling observer who, on gettingback to Earth after aspacecruise in space, turns out to have aged less than everyone at home. The differenceinthe overall elapsed durations can be derivedfromthe basic equations of Special Relativity theory:itdepends on the waythe travellingobserver is accelerated, as wellas on the speed at which he is propelled across spaced uringh is round-trip. Generalizingt he lesson, twoacceleratedclocks measuredifferent proper times along their respective journeys, even if the intervalunder consideration is bounded by the same pair of events (separation, reunion). Foracomplete genealogy of "Langevin'sparadox" fromEinstein to Bergson (through vonLaue, Weyl,a nd Painlevé), see During( 2014). tion is that the genuine issue behind the Bergson-Einstein dispute crystallizes in this simple question: in what sense aret he twins contemporaneous? Fors urely, they are contemporaneous in some sense. There mayb en os uch thing as "the" durationo ft heir separation, but whys hould we view them as temporally insulated from each other,each locked in his own proper duration, so to speak? Which in turn raises the following question: if we resist this form of temporal solipsism, if we acknowledge as ense of contemporaneity allowing the twins to be temporallyr elated beyond the familiar figures of globali nstantaneity and local coincidence, how does this reflect on the coexistence of each of them with the rest of the universe? Forinthe absence of an overall physical connecting medium (aether or otherwise), it seems as if we were left once again with the formal aether of space-time as the sole factor of unity.Shouldwesay that the temporalsense of cosmic unity can onlybeachieved from place to place, rather than in one stroke? But then how is it possible to overcome the limitations inherent to proper time? How can we recover asense of temporal depth and perspective without once again framing time?
The truth is thatp hilosophical reflection finds itself in ad ifficult position: standing halfwayb etween locality and totality,w ith no clear sense of what could constitutei ts proper frame of reference, it is confronted with aweb of interlockingdurationss omehow surveying each other temporallyb yt he mere fact of belongingt ot he sameu niverse. The exactn atureo ft his reciprocal survey is what is at stake here, and it need not be formulated from the outset in metrical terms.For the twins separate onlytomeet again, and surelyitmakes sense to say that while the traveller was away,cruising in space, his brother on Earth gotd ivorced and remarried, whatever the durations elapsed on either side. The twins mayturnout to have aged differently, but this does not prevent them from being contemporaries all along,t hroughout their separation. This much is certain, at least in retrospect.I tremains to be seen what is involved in this tenseless statement: the twins coexist as they go about their business along separate spatiotemporalr outes. How can we confer genuine temporals ense to such ac laim? Bergson'sa ppeal to real time takes on its full meaning in this context: Thus the "unity of real time" is confirmed by the "the simultaneity of flows"-which Bergson contrasts with the "simultaneity of instants"-,a nd more cogentlyt han anyc onsideration regarding the metrical equalityo fp roper Time as Form: Lessons from the Bergson-Einstein Dispute times. Considered in this light,Langevin'sspace-age scenario presents us with a theoretical toy-modelfor addressingamore general issue that is cosmological at its core. In fact,i tc an be argued thatt he twins' story cannot even be meaningfullytoldifitisnot playedout from the outset against acosmic backdrop, rather than having them hang in abstract space-time as if nothing else existed. The traveller twin, as Whitehead and others have rightlypointed out in Machian fashion, ages less because his personal involvement with the universe as aw hole is different from that of his stay-at-home brother (Whitehead 1923,35).¹⁹ This shows in the fact thath ei ss ubjected to inertial forces in the acceleration phases of his journey,w hile the other is not.A dmittedly, Bergson'sr epeated claim thatt he twins must nevertheless find themselves, once reunited, having aged the same, did not do much to clarify the matter. But the stubbornness with which he attempted to refutet he very premise of the paradox wasi nstrumental in bringingo ut certain aspects of the situation that are too easilyo verlooked. Chief among them is the question of the exact rangeo ft he twins' perspectives on the "wave" of becomingt hat carries their respective flows of duration. If these flows are commensurable (which they are, at least in the sense that their respective proper timesc an be compared), to what extent can they be synchronized?( Fort hey can, at least in the limited sense whereu nilateral and relative simultaneity relations can be defined on each side). If there is no way of achieving consistent and continuous overall synchrony, in what sense do the twins share acommon history?Are figures such as waves and sheafs suitable to describet he process in which they participate, knowing that the perspectival view taken by accelerated observers induces constant disruptions and shearsi n the account of elapsed durations?W hat is the exact locus of the relational present that the twins seem to share despite theirdiverging proper times?Finally, is the philosopher in ab etter position than the physicist for assessing the situation?²⁰ There is probablyn ou nivocal answer to such questions,b ecause coexistenceitself comes in aplurality of modes or regimeswhich appear to be embedded and somewhat superimposed within space-time itself. But it is difficult to ignore them altogether.S implyp ut,t hey stem from the sense thatt he twins  See French 1968, 156: "Would such effects as the twin paradoxexist if the framework of fixed stars and distant galaxies weren ot there?"  Bergson believes that this is the case, because the philosopher,who does not caremuch for actual measurement,isfreetodowithout reference frames-leaving them to their mutual, reciprocalmotion, overviewingthe scenefromnowhere,sotospeak. It is as if aprivilegeofextraterritoriality allowed him to describe mirroring perspectiveswithout havinghimself to choose any viewpointi np articular. are indeedc ontemporaneous, although they account for this fact in different ways.I nt he quotation givena bove( " they are contained in the samei nterval"), Bergson likens the "time" elapsed between the moments of separation and reunion-atime which Langevin shows to be measured differentlybyeach-to athick interval of extended present that they both share within what mayb ec alled an interval or region of contemporaneity.
This can be givenp recise topological meaning in the space-time framework (Čapek 1971,2 48 ff), provided thatw ed on ot forgett hat the disjoint space-time paths of the twins remain generallyi ncommensurable as far as standard simultaneity is concerned. Forintroducing an inertial frame somewhereinthe picture can yield no more thanarelative and arbitraryperspective on the overall simultaneity of their unfolding durations: it is frame-time once again. There is no point denying the relativity of simultaneity defined in such an arrow sense, i. e., as a "simultaneity of instants".B ergson consistentlyd ownplays its philosophical relevance because he is convincedt hat instantsa re unreal-ideal constructs, just as the frames themselves. No wonder that simultaneity relations between mere mathematical fictions should provet ob er elative… The best one can say is that ac ontinuous one-to-one correspondenceb etween simultaneous events on bothp aths is available in some frames. This is alreadys omething,b ecause as it happens the very fact thatf rame-time and globals imultaneity relations are available in some frames is itself an absolute( frame-invariant) fact about the situation-af act that mayt urn out to be more significant,a sf ar as the "unity of real time" is concerned, than the discrepancy between elapsed proper times.
Thus, the Earth twin, occupying asingle frame, can "sweep along" the traveller'spath, plottinghis distant proper time against his own from one instant to another.The resulting account of the traveller'se lapsed time is necessarilyr elative to the choice of the Earth-bound reference frame:there is nothing absolute, nothing real in the kind of simultaneity achieved from such frame-time. Asymmetrical attempt from the accelerated twin would necessarilyr esulti ng aps, blind-spots and temporalj ump cuts, exacerbatingt he sense of disjunction and separation that is most likelyi nherent in anyr elation of simultaneity at ad istance.²¹ However,this mutual framing of the shared zone of contemporaneity between the twins can be complementedbyacontinuous exchangeofelectromagnetic signals between the twins (factoringi nD oppler effects), allowing each to form aconcrete and continuous-though delayedand distorted-imageofhis co- Foradiagrammatic account of this oddity stemmingfromthe metrical structureofrelativistic space-time, see Whitehead (1923).
Time as Form: Lessons from the Bergson-Einstein Dispute existencew ith the other.L angevin'so riginal scenario introduces this additional twist.B yo pening al ives tream of information between the twins, am easure of connectedness and continuity is restored within relativistic simultaneity.

9T he TwinsI I: Contemporaneity and the Active Present
The temporal perspective introducedbysuch real-time communication is essentiallyd ifferent from the one classicallya ssociatedw ith referencef rames, where simultaneity relations applyt od istante vents thata re by definitionc ausally insulated (space-likes eparated) from each other.I th elps us realize, by contrast, what is reallyi nvolvedi nt he relativization of simultaneity defined in terms of instantaneous planeso fs imultaneity.Whitehead wasp erceptive enough to generalize the situation based on purelyt opological considerations. Drawing from the light-cone structure of relativistic space-time, he devised an elegant definition of "contemporary events":c ertain pair of events are indeterminate as to their time order simply because their mutuall ocations in space-time prevent them from influencinge ach other.I no ther words, an exchangeo fs ignals between them would have to be faster thant he speed of light.S uch events are said to stand in ar elation of mutual causal independence. This simple definition is also found in Reichenbach'sc ontributions to the philosophyo fs pace-time. One of its advantagesi si ts universal scope: for any givene vent with its associated light-cone, the set of its "contemporaries" coincides with the set of events laying in the wedge-shaped region outside the cone. The form of the causal nexus thus appears hollowed out through and through: it is as if each event brought with it an egative nexus,t he shadow cast by all thati sc oncealedf rom it.T his outer zone of contemporaneity, which Eddington called the "AbsoluteE lsewhere",i ss ometimesr eferred to as the "topological present" in the current literature on space-time coexistence (see, e. g., Balashov 2010,68). It illustratest wo essential facts about simultaneity:a )r elations of simultaneity are basedo nf acts of causal disconnection, and b) they extend to thick regions of space-time, rather than being confined to infinitelyt hin layers of instantaneous coexistence.²² Bergson alreadyr ecognized that the simultaneity of instants finds its condition in the simultaneity of flows. Whitehead goes further,s howing that for anyt wo contemporary events,  In Eddington'sterms: "the absolute past and future arenot separated by an infinitelynarrow present" (Eddington 1929,48). there willbesome reference frame in which they are simultaneous in the usual, Einsteinian sense (Whitehead 1925,77). Hence, the relativity of simultaneity can be reformulated in terms of the degrees of freedom we enjoy in slicing at different angles across the zone of contemporaneity attachedtoagivenevent.The resultingplanes of simultaneity are so manyperspectivestaken on amore comprehensive region of contemporaneity.H encet heir inherent relativity takes on objective meaning:itisanexpression of the temporalunderdeterminationofdisconnected events, as much as of the arbitrary choice of referencef rames.
Compelling as it is, the interpretation of coexistence as contemporaneousness has some limits: it is restricted to certain classeso fe vents (those that are space-like separated), and more importantly,from apractical standpoint it is ultimatelyr elative to the point-likep erspective openedb yp articularp oint-events in space-time, rather thanspace-time paths or stretches of duration. As aresult, it is not easilya pplied to real enduringobservers and more generally, extended processes. Nevertheless, the negative definition of coexistence in terms of disconnection or separation manages to captureabasic phenomenological feature of our extended present that is best illustrated by the experience of somewhat helplessly waitingfor the answer to amessage.²³ It is as if asiphon weredraining the time elapsingemission and reception, creating asense of absence and void.²⁴ This sheds light on the twins' scenario. Fort he twins tooa re separated while contemporaneous. In their case, the element of disconnection (in space) is dialecticallyi ntertwined with that of connection (in time). Absence is incorporated within an overall sense of distended co-presence. FollowingWhitehead, we may saythat the situation typically "expresses how contemporary events are relevant to each other,and yetp reserveamutual independence. Thisr elevance amid independence is the peculiarc haracter contemporaneousness" (Whitehead 1958, 16). The point,h owever,i st hatt he twins' separation is not absolute: the twins qua living observers endure;b esides pairs of contemporary events on theirr e- This point is nicelyi llustrated by Eddington: "Suppose that youa re in lovew ith al adyo n Neptune and that she returns the sentiment.Itwill be some consolation for the melancholyseparation if youc an sayt oy ourself at some-possiblyp rearranged-moment, 'She is thinkingo f me now'.Unfortunately,ad ifficulty has arisen because we have had to abolish Now.There is no absolute Now,but onlythe various relative Nows differingaccording to the reckoningofdifferent observers and coveringt he whole neutral wedge which at the distanceo fN eptune is about eight hours thick. She will have to think of youc ontinuouslyf or eight hours on end in order to circumvent the ambiguity of 'Now'" (Eddington 1927,4 9).T he "neutral wedge" refers to the wedge-shaped neutral zone between twol ight cones: the intersection of their respective outerz ones of contemporaneity.  Sartrehas provided compelling phenomenological elucidations of this experienceofseparation (see During2 018, 423 -425).
Time as Form: Lessons from the Bergson-Einstein Dispute spective paths, there are innumerable events which can in fact be causallyconnected, as illustratedb yL angevin'sh ypothesis of communicating observers. Thus, the various schemas of coexistence appear subtlyentangled. As they continuouslyexchangeelectromagnetic signals,the twins coexist in the sense Bergson spoke of asimultaneity of flows, but in other respects they are contemporaneous with each otherinthe sense Whitehead spoke of the mutual relevance of independent events.
Other models of non-standard simultaneity suggest themselvest om ake sense of the distended coexistence of the twins. Taking one further step in the direction of co-presence, we mayc onsider the active( or interactive) present based on the so-called "Alexandrov interval",d efined by the intersection of the future light cone of an event Awith the past light cone of an event Bcausally related to A.²⁵ Within this diamond-shaped region of space-time, all events can be causallyrelated to bothAand B. Thus, if Aand Bare two events punctuating the worldline of an observer, the interval defines az one of active present comprising all the entities, objects, processes with which this observer can interact duringashort but finite interval of proper time such as the one corresponding to the specious present.This seems rather intuitive,f or the objects with which we can interact within the bounds of our specious present certainlyc ontribute to our perception of afield of co-presence in which we participate with other beings. Each of the twins carriesw ith it such an interval of active present.B ut to properlya pprehend their coexistence requires that we paya ttention to the patterns of intersection between theirr espective presents.F or observers coexist in ar elevant sense when their active presents substantiallyo verlap, outlining a specific zone of co-presence thatexpresses the particular nature of their relation. (Incidentally, in the caseo fa symmetrical relations,coexistence mayt ake aunilateralform, distinguishing itself from the common understanding of simultaneity relations as reflexive,symmetric and transitive.) What was introduced earlier as aregionofsimultaneity-atopological envelope defined by two doublyintersecting worldlines-can now be redescribed as af ield of relational coexistence, provided that observers involved in that field interact in asymmetric wayduring the entire time of their separation. It is thus possible to account for the twins' story in away thatisboth frame-independent and conjunctive,offering aunified picture of their shared history,n otwithstanding the amount of temporald istortion and disruption induced by the underlying dynamics.
Kant'sdoctrine of simultaneity in the ThirdAnalogy of Experience followed a similar pattern: the relational theme was givenbythe category of community or  On this and other issues of spatio-temporal coexistence, see Balashov 2010,1 43 ff. reciprocal action. Yet, despite the claim that simultaneity is a sui generis temporalrelation that cannot be reduced to the non-successive,the positive meaning of that relation remained somewhat obscure. To give substance to simultaneity,the best Kant could do was to refer it to the sheer densityofthe links of mutualcausal dependence between enduringobjects. In true Leibnizian fashion, coexistence came in the formo fas eamless plenum of interactions. By contrast,e mbracing the philosophical consequenceE instein'sp rinciple of locality-the idea that in the absence of instantaneous action at ad istance, everyc onnection takes time -,Whitehead'sa pproach acknowledgest he primordialf unction of causal separation, bringingt ol ight the negative nexus embedded within relativistic spacetime. In that respect,c ontemporaneity is the obverse of simultaneity.T aken together,t hey form ad ual imageo fc oexistence, giving it its full scope.

The TwinsI II: Zeno'sS hadow
This survey of some varieties of coexistenceserved one main purpose, namely to drive home, once again, arather simple message: we are not dealing with time, properlyspeaking, unless we make room for all its relevant dimensions, including simultaneity in the generalized sense just considered. The value of the twin paradoxr esides in the simplicity and generality of the situation from which it arises: it forces us to re-examine our ideas about coexistence. Reflecting these ideas against space-time, interpreting them in the light of ac ategorial scheme that physics itself does not provide, reveals an intricate and multi-layered dialectics of local and global, invariancea nd perspective,connection and separation.
The truth, however,i st hat Bergson argued his caseq uite differently. He made it seem as if he wast rying to preservea ta ll costs,i na nu ncommonly a priori mannera nd for essentiallyc onservative purposes, the sheer equality of the twins' elapsed durations. This assumption of metrical uniformity directly contradicted one of the tenets of Relativity theory,s ince the synchronicity of proper times is not preserved in the general case involving acceleratedobservers. More serious still, it obscured the underlying issue of coexistence by virtually aligningthe entiresituation on the trivial case of two co-moving inertial observers.
Af ew hypotheses mayb ev entured as to the reasons behind Bergson'sm isguided tenacity.The first thing to consider is simplythe immediate benefit of refusing to acknowledge the difference in overall aging. Bergson realized that it was the most straightforward wayo fp reservingasense of temporalu nity and shared human experience, while remainingf aithful to the metaphysical views set forth in his earlier works (Bergson 1991, 209 -211). The metrical uniformity of time measurement was immediatelycompatible with the idea of an essential rhythmic uniformity of both matter (the most relaxed degree of duration) and human consciousness (characterized by its own specific degreeo ft ension). Since the metaphysical grounds of this temporaluniformity werenot directlydiscussed by Bergson in the context of Relativity,itwas difficult to resist the impression thatt he philosopher was merelyc lingingt os ome intuitive and ultimately subjective concept of absolute time.
Whydid Bergson laysom uch stress on metric equality,when all he needed to establish was the somewhat looser connection between real time and the generic uniformity of livedd uration acting as ac onnectingt hread between dispersed flows of duration exhibitingvarious degrees of tension?Toclarify his motives, it is important to bear in mind the basic insight behind the battery of arguments devised to expose the unreality of the temporal perspective effects underlying Langevin'sp aradox. These arguments can be traced backt oa nother paradox. The "Stadium",a lso known as the paradox of the "Moving Rows",i s arguably the least famous among Zeno'sp aradoxeso fm otion. YetB ergson deems it the most instructive (Bergson 1991, 192).T he classic version involves bodies (rows) of equall ength moving along parallel tracks within as tadium, at different speedsa nd in opposite directions. If Aristotle'sa ccount in Physics VI, 9i st ob et rusted, Zeno fallaciously argued that,g iven the appropriate speed ratio, the elapsed duration attached to ap articularm oving bodyw ould appear to be double of itself when measured by the trace left along another bodym oving at ad ifferent speed. It is easy to seet hat we are dealing here with reference frames in relative motion. In this regard, the pages devoted to "light figures" in Duration and Simultaneity,c hap. V, while containing no direct mention of the "moving rows",o ffer as triking parallel with Zeno'sp aradox. Bergson substitutes for the moving rows aray of light moving backand forth between two plates-as ituation thats hould be familiar to anyone who has been introduced to Relativity theory by means of considerations regarding the behaviour of "light clocks".Viewed from different reference frames moving at various speeds, the light figure traced by the rayoflight will appear variouslyslanted or distorted, it will exhibit shapes of different lengths-all equallyvalid spatial projections of one single time lapse.
Based on this example, Bergson interprets relativistic effects such as length contraction and time dilation as mathematical artefacts stemmingfrom the conditionsofmeasurement,more particularlyfrom the correlation of all elapsed durations with trajectories in space. Since the spatial expressions of duration undergo deformations through the prism of speed, durations themselvesa dmit as manyv alues as there are degrees of speed-in fact infinitely manys ince reference frames can be arbitrarilyc hosen in order to track light.R elativity,i nt hat sense, offers ac oherent theory of the changingk inematicp erspectiveso ne may take on real motion and duration: the Lorentz transformationsa ccount for the resulting perspective effects while giving mathematical expression to what remains invariant under the virtuallyinfinite multiplication of dilated times. Bergson rightlyemphasizes the invariance of proper times beneath the kaleidoscopic deformations of improper times; but as far as duration itself ("true duration"), the measuring operation onlyt ouches its surface. The internal changea ffecting matter remains indifferent to its spatial projections under perspective views. The "unity of real time" is thereby preserved, although mathematicallyt his may seem to boil down to the invarianceoflocal time which, as we have seen before, cannot be the last wordonthe matter.A tthis point,Bergson'sstrategyseems to break down. But his diagnosis,d elivered as ad istant answer to Zeno'sa rguments, remains valid as long as we are dealing with uniformmotions and inertial frames.
The more pressingp roblem is to understand how this reflects upon the discussion of Langevin'sparadox, which involves accelerated observers. Quite simply, fascinatedashewas with the relativistic transposition of the Stadium, Bergson was led to systematicallyo verstate matters of symmetry,p erspective and relativity in the more general case illustratedb yt he twin paradox. As Zeno's shadow was cast over the twins, he was led to believet hat the paradoxc ould be diffused as yeta nother instance of purelyp erspectival effects. That is why he insisted that ar igorous formulation of Langevin'ss cenario should maintain acompletesymmetry between the twins' space-time trajectories, each being entitled to takeh imself to be at rest,while the other is in motion.
What about the obvious geometricalo bjection?I no rder for the twins to eventuallym eet again, one of them must make aU -turn at midcourse. Regardless of who is actuallyt ravelling,adissymmetry is bound to occur somewhere, at some point,r esulting in an overall differencei ne lapsed durations. As previouslys tated, the longest routet hrough space-time happens to be the shortest through time:²⁶ Bergson did not realize the full implications of this basic mathematical feature of Minkowski space-time, because he systematicallyd ownplayedt he importance of space-time constructions,which he viewed at best as mathematical devices with no real ontological grounding. He generallybelieved  In the idealized "3-clocks version" of the twin paradoxw here the round-trip involves only uniform motions,t his metrical oddity is clearlye xhibited by the relativistic counterpart of the moref amiliar Euclidean "triangle inequality".T he sum of the lengths (i.e., elapsed proper times) of the oppositesides of atriangle drawninMinkowskian space-time is shorter,not longer. Hence the idea that crooked paths in space-time constitutet emporal "shortcuts".S ee During 2007,99-100. Time as Form: Lessons from the Bergson-Einstein Dispute he could playt he physicist at his own game by dealing with the paradoxi na strictlyrelational manner, de facto abstracting from all the relevant physical features of the situation. By neglectingthe dynamic aspects of the situation in favor of the kinematic reciprocity of the observers' perspectivesu pon their respective trajectories and timelines, he reducedthe paradoxtoamere thought experiment, an argument to be dealt with on purelyconceptual grounds.Hemade it seem, in short,a si ft he task of plotting the twins' relative motions in space-time was essentiallyunderdetermined,allowing for multiple equivalent spatio-temporal embeddings. Once the twins wereconstrued as interchangeable, theirrespective durations could onlye nd up coinciding.²⁷ In Bergson'sd efense, it is based on similar premises thatP aulP ainlevé,a first-class mathematician and member of the French government, had boldly challenged Langevin (in 1921), and later Einstein himself in 1922. The latter episode took place on April 5, one daybefore Bergson'smeeting with Einstein (Bergson 2009,402 -409). It provides us with another test-casefor the principle of hermeneutic symmetry.T om ake his point,P ainlevé had devised an even simpler model thanthe original: Langevin'sJ ules-Vernesque rocket and its space journey had been replaced by at rain leaving its station to make ar ound trip. More importantly,t he story involved onlyc onstant velocities, suggesting ap erfect symmetry or reciprocity between observers in relative,u niform motion. From there, Painlevé argued that time dilations being reciprocal, their effects should simplyc ancel out.
Einstein easilyoverturned the objectionbyremindinghis eminent colleague that the situation he was describing did not in fact involvet wo frames of reference in relative motion, but three. By the mere fact of making aU -turn to come back to its starting point,the train observer was forced to "hop" on anew reference frame at midcourse. Therein lies the reason for the overall discrepancy in elapsed times. Painlevé immediatelyc onceded Einstein'sp oint and the matter was thus settled to the satisfaction of all parties. Understandably, Bergson did not seefit to take up the matter again the next day, when his turn came. Instead, he chose to deliveralecture on simultaneity.Painlevé's5-minutes argument with Einstein nevertheless left adurable trace on him, as attested by the fact that it is literallyr eproduced (and dulyc redited) in Duration and Simultaneity,a nd dis- The strategyisreminiscent of the wayBerkeley,Mach or Poincaré criticized Newton'sabsolute spacebyusingthe symmetries of aphysical situation to establish the actual indiscernibility of two stateso fa ffairs.Thus,i ft he universe were reducedt ot wo particles in relative motion, therew ould be no wayo ft elling which particle is really accelerated, or directlya ffected by time dilation. The two would be literallys ubstitutable, so that anythings aid about the one could just as easilyb es aid about the other. cussed again at length in the appendices. Clearly, it must have had some philosophical merit in his eyes, despite the fact that it had been refuted. But there is little to be gained in defending the indefensible. With the benefit of hindsight,it cannot be denied thatthe more relevant issues regardingcontemporaneity were obscuredb yB ergson'ss tubborn insistenceo ni nterpreting the twins' paradox through the lens of time-dilation, in terms of referential and reciprocal effects. Relativity,i nt he broad sense Poincaré gave to this term when speaking of the relativity or homogeneity of space (i. e., the symmetries accounting for the similarity of figures), certainlyf unctioned as an epistemological obstacle in that respect.S od id the projective metaphor of perspective underlying the criticism of so-called "fictitious times".

Conclusion
These elements of context mayhelp us better appreciate, by contrast,the ongoing relevanceofBergson'sotherwise frustrating debate with Einstein. Like several scientists and philosophers of his time, he certainlyf ailed to appreciate the structural relevanceo ft he twin paradoxf or Relativity theory.Thisb lindspot in his assessment of relativistict ime is palpable in the resistance he opposed to the idea of unsyncable durations, and moreg enerallyt ot he notion of local time. But the different circumstantial reasons reviewed in this paper should not overshadow the more fundamental ones, chief among which is ad eepa ttachment to the idea of time as form, despite the emphasis on heterogeneous durations and rhythms. On the upside, from the commentator'sp erspective,B ergson'sq uasi-intentional "blunder" and the discussions it triggered provide an opportunity to clear the ground and allow vital questions to emerge in plain sight.The sublimatedv ersion of the twin paradox, unfolding in abstract homogeneous space, plainlydistorts Langevin'soriginal intent,but by doing so it also directs our attention to the fact that the lines of flow of extended matter,refracted and dispersed as they are throughout the universe, goingout of sync at every moment,s tilld os otogether in ag enuine temporals ense. These flows are contemporaneous, and in more than one way. Simultaneity does not reduce to absolutef acts of spatio-temporal coincidenceo rt ot he conventional framing of world-wide instants: therea re such thingsa ss heaveso fs imultaneity.T he twins illustrate this basic truth in their own inchoate manner.R ealizing it opens up new perspectiveso nt he problematic temporalu nity of material process. This process maywell turn out to be fundamentallyopen at the cosmological level because the universe itself endures and is subject to change, but this should not prevent us from trying to make sense of the unity of material dura-Time as Form: Lessons from the Bergson-Einstein Dispute tions. The samen aturallyh olds true of the living in general. Thec hallenge, in every case, is to approach this unity in temporalt erms, sub specie durationis. What distinguishesB ergson'sv ersion of time form in that regard is that none of its concrete models can be achieved in one stroke: they are themselvesi n the making.