Full Idea
Falsity is the assertion that that which is is not or that that which is not is, and truth is the assertion that that which is is and that that which is not is not.
Gist of Idea
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
Source
Aristotle (Metaphysics [c.324 BCE], 1011b20)
Book Reference
Aristotle: 'Metaphysics', ed/tr. Lawson-Tancred,Hugh [Penguin 1998], p.107
A Reaction
It was very startling to discover Plato's Idea 13776, and realise that this famous and much-quoted idea of Aristotle's was not original to him. I find it very hard to disagree with any aspect of the idea.
Related Ideas
Idea 13776 Truths say of what is that it is, falsehoods say of what is that it is not [Plato]
Idea 23298 Neither Aristotle nor Tarski introduce the facts needed for a correspondence theory [Davidson]