Wikipedia 10K Redux by Reagle from Starling archive. Bugs abound!!!
A proposition and its denial cannot both be true at the same time and "in the same respect." In other words: for any proposition P, it is not the case both that [P] is true and that [not-P] is true. This is closely related though distinct from TheLawOfNoncontradiction and TheLawOfExcludedMiddle.