 Elsewhere, in Metaphysics (998b22), Aristotle argues explicitly that there cannot be a highest genus (e.g., of being or unity) shared by entities of different categories (cf. Ackrill 1963, 81). For a species is defined in terms of its subsuming genus and differentia (e.g., man is definable as an animal that is rational), and while the genus (animal) may be predicated of the species (man), it may not be predicated of the differentia (rational). As a result, if being (or unity) were a genus, no differentiae could be said to have being (or to be one); but “the differentiae of any genus must each of them both have being and be one” (Metaphysics 998b22–3).