Full Idea
There are two sorts of one. There is the one which is convertible with being, which adds nothing to being except being undivided; and this deprives of multitude. Then there is the principle of number, which to the notion of being adds measurement.
Gist of Idea
'One' can mean undivided and not a multitude, or it can add measurement, giving number
Source
Thomas Aquinas (Quaestiones de Potentia Dei [1269], q3 a16 ad 3-um)
A Reaction
[From a lecture handout] I'm not sure I understand this. We might say, I suppose, that insofar as water is water, it is all one, but you can't count it. Perhaps being 'unified' and being a 'unity' are different?