   Ζ.12 introduces a new problem about definitions—the so-called “unity of definition.” The problem is this: definitions are complex (a definiens is always some combination of terms), so what accounts for the definiendum being one thing, rather than many (1037b10)? Suppose that man is defined as two-footed animal; “why, then, is this one and not many—animal and two-footed?” (1037b13–14). Presumably, Aristotle has in mind his discussion in Ζ.4 of such “accidental unities” as a pale man. The difference cannot be that our language contains a single word (‘man’) for a two-footed animal, but no single word for a pale man, for Aristotle has already conceded (1029b28) that we might very well have had a single term (he suggests himation, literally ‘cloak’) for a pale man, but that would still not make the formula ‘pale man’ a definition nor pale man an essence (1030a2).