This article discusses the ontological status of the parts of substances in Aristotle’s theory of categories. Since in this theory the substantial parts, as well as the wholes, are primary subjects of predication, i.e. substances, a part may be to its whole as Socrates is to Callias so that the mereological composition of substances can be regarded as an accident (a relation). It is argued here that in Cat. 7 Aristotle is trying to rule out this possibility by redefining the boundaries of the category of relatives itself. Starting from the framework of the Academic debate and then following closely the text of Cat. 7, the author provides a detailed reconstruction of Aristotle’s argument in order to establish the reason for which the parts of substances hold such an uncertain status. Furthermore, he shows that the Categories do not provide a definitive solution to the mereological problem, which has rather to be sought in the Metaphysics. The following results are attained: a moderately systematic component is revealed in Aristotle’s Categories and a mereological element is detected in the very core of his theory of substance.
Keywords: Aristotle, category, relation, substance, part, definition