TheLawOfNoncontradiction

Variously formulated, the gist of the Law of Noncontradiction is that a proposition and its denial cannot both be true at the same time and "in the same respect." If we're being careful, we won't formulate it this way, in terms of truth. Instead, we'll say the law is: for any proposition P, it is not both the case that P and not-P.

The Law of Noncontradiction is almost universally affirmed by philosophers. See also TheLawOfExcludedMiddle and ThePrincipleOfBivalence.