Full Idea
The most certain of all basic principles is that contradictory propositions are not true at the same time.
Gist of Idea
The most certain basic principle is that contradictories can't be true at the same time
Source
Aristotle (Metaphysics [c.324 BCE], 1011b13)
Book Reference
Roochnik,David: 'The Tragedy of Reason: the Platonic logos' [Routledge 1990], p.149
A Reaction
Principle of Noncontradiction