Full Idea
But this sum now - isn't it just when there is nothing lacking that it is a sum? Yes, necessarily. And won't this very same thing - that from which nothing is lacking - be a whole?
Gist of Idea
A sum is that from which nothing is lacking, which is a whole
Source
Plato (Theaetetus [c.364 BCE], 205a)
Book Reference
Plato: 'Complete Works', ed/tr. Cooper,John M. [Hackett 1997], p.227
A Reaction
This seems to be right, be rather too vague and potentially circular to be of much use. What is the criterion for deciding that nothing is lacking?