Galen’s elemental mixture of fire, air, water and earth (and of the corresponding primary qualities, hot, cold, dry and wet) is primarily a physical process, in which primary elements mix and give rise to all compounded physical bodies, inanimate and animate. As such, the concrete, physical process of mixture is an essential basis for a thorough understanding of Galen’s physical system. In this article I pursued a twofold aim. First, I showed Galen’s syncretic approach while expounding his theory of mixture (insofar as he attributes that theory equally to ‘Hippocrates’, Aristotle and the Stoics) and illustrated the reasons behind this approach. Second, and more importantly, I addressed Galen’s epistemological position concerning mixture’s constituents and answered the question of whether, according to Galen, are the qualities alone, conceived more Aristotelico as conceptually distinct from the body itself, to mix or if Galen viewed mixture, as the Stoics did, as a coextension of bodies. As we argue, Galen assigns the epistemological status of πιθανόν to the Aristotelian and Peripatetic doctrines. This indicates that, to a certain extent, he joins this historical debate and is more closely aligned with Aristotle and the Peripatetics than with the Stoics. By resorting to what is πιθανόν Galen obtains two results: (1) does not expose himself to accusations of dogmatism but, at the same time and in his method, enters the ongoing controversy about qualities and bodies without taking a definitive position, and (2) he preserves his syncretic theoretical structure which finds its linchpin in the defence of a medical-cum-philosophical continuism.