Full Idea
Those statements that say of the things that are that they are, are true, while those that say of the things that are that they are not, are false.
Gist of Idea
Truths say of what is that it is, falsehoods say of what is that it is not
Source
Plato (Cratylus [c.375 BCE], 385b)
Book Reference
Plato: 'Complete Works', ed/tr. Cooper,John M. [Hackett 1997], p.105
A Reaction
It was quite a shock to discover this, because the famous Aristotle definition (Idea 586) is always quoted, and no modern writers seem to have any awareness of the Plato remark. Classical scholarship is very poor in analytic philosophy.
Related Idea
Idea 586 Falsity says that which is isn't, and that which isn't is; truth says that which is is, and that which isn't isn't [Aristotle]