Full Idea
What first comes to mind is being; secondly, that this being is not that being, and thus we apprehend division as a consequence; thirdly, comes the notion of one; fourthly the notion of multitude.
Gist of Idea
Being implies distinctness, which implies division, unity, and multitude
Source
Thomas Aquinas (Summa Theologicae [1265], I Q11 ar2 ad4), quoted by Roderick Chisholm - Person and Object 1.5
Book Reference
Chisholm,Roderick: 'Person and Object' [Open Court 1976], p.36
A Reaction
This is one of the best things I have read on 'being'. It is the Aristotelian recognition that we can only study being by studying identity, and that this leads on to wider metaphysics. Other approaches to being are dead ends.