 Aristotle’s first system of classification is of beings, (τὰ ὄντα) (1a20). The division proceeds by way of two concepts: (1) said-of and (2) present-in. Any being, according to Aristotle, is either said-of another or is not said-of another. Likewise, any being is either present-in another or is not present-in another. Because these are technical notions, one would expect Aristotle to have defined them. Unfortunately, he does not define the said-of relation; and his definition of the present-in relation is either circular or rests on an undefined concept of being in. He says: ‘By “present in a subject” I mean what is in something, not as a part, and cannot exist separately from what it is in’ (1a24–5). Notice that the word ‘in’ occurs in this definition of present-in. So, either ‘in’ means the same as ‘present-in’, in which case the definition is circular; or ‘in’ is itself in need of a definition, which Aristotle does not give. Hence, Aristotle’s first system of classification rests on technical concepts whose precise characterization is not settled by anything Aristotle says.