In addition to the syntactic (inflectional) causative suffix -(s)ase- , Japanese displays a lexical (derivational) causative suffix -(a)se- ( okur-ase- ‘delay’, no-se- ‘place on’) whose a- zero alternation is general in the stem-level phonology. Because the UR of syntactic -(s)ase- must include the s of its postvocalic alternant, the two suffixes necessarily have distinct phonological forms; there is thus no way to treat them as “high attachment” and “low attachment” versions of a single element. The division of labor between syntactic -(s)ase- and lexical -(a)se- invites the conclusion that no causative suffix is syntactic in some cases and lexical in others; if so, causatives pose no challenge to the position that while Japanese inflectional morphology is the phonological realization of syntactic representations, derivational morphology involves lexical listing of stems. The conclusion that there is no causative suffix that spans the syntactic/lexical boundary is validated by showing that forms that have been taken in the literature as exemplifying lexical -(s)ase- are in fact either lexical -(a)se- or syntactic -(s)ase- . As part of this demonstration, it is shown that, with minor exceptions, verb stems in -ase- have arisen as variants of pre-existing stems in -as- , and an explanation for this ongoing process of replacement is proposed.