 An examination of Stoic ontology might profitably begin with a passage from Plato’s Sophist (cf. Brunschwig 1994). There (247d-e), Plato asks for a mark or indication of what is real or what has being. One answer which is mooted is that the capacity to act or be acted upon is the distinctive mark of real existence or ‘that which is.’ The Stoics accept this criterion and add the rider that only bodies can act or be acted upon. Thus, only bodies exist. So there is a sense in which the Stoics are materialists or – perhaps more accurately, given their understanding of matter as the passive principle (see below) – ‘corporealists’. However, they also hold that there are other ways of appearing in the complete inventory of the world than by virtue of existing. Incorporeal things like time, place or sayables (lekta, see below) are ‘subsistent’ (huphestos, Galen 27G) – as are imaginary things like centaurs. The distinction between the subsistent and the existent somewhat complicates the easy assimilation of Stoicism to modern materialism.  It’s not wrong to say that all existent things are corporeal according to the Stoics, but one needs to add that existent things don’t exhaust their ontology. All existent things are, in addition, particulars. The Stoics call universals ‘figments of the mind’. Platonic Forms, in particular, are rejected as ‘not somethings’ which lack even the subsistent status of incorporeals like time, place or sayables (Alexander, 30D). The Stoics’ positive nominalist alternative is harder to interpret. Some texts suggest that they offered a conceptualist treatment akin to Locke’s, treating an apparent predication like “man is a rational, mortal animal” as the disguised conditional, “if something is a man, then it is a rational mortal animal” (Sextus Empiricus, 30I). But there may well have been development within the school from this conceptualist view toward a form of predicate nominalism. See Caston (1999).