Full Idea
Things are numerically one in matter, formally one in their account, generically one in their pattern of predication [genos], and one by analogy if related to a further one.
Gist of Idea
Things are one numerically in matter, formally in their account, generically in predicates, and by analogy in relations
Source
Aristotle (Metaphysics [c.324 BCE], 1016b30)
Book Reference
Aristotle: 'Metaphysics', ed/tr. Lawson-Tancred,Hugh [Penguin 1998], p.124
A Reaction
Very subtle distinctions. What I like is that the notion of numerical unity is comprehensively tied to the notion of individual identity. 'To be is to be countable' may be wrong, but it is better than Quine's 'to be is to be the value of a variable'