Latin principle of debate, stating there is no valid argument against a fact.

It is an important principle, as sometimes logic may result in incorrect conclusion, as in the classical Greek argument that Achilles may never catch up with a turtle because to get to it, he must first walk half the distance, then half the remaining distance, etc. (This argument is easily disproved with modern mathematics, but presented an unsolvable paradox in ancient times.) But since it is a fact that Achilles can indeed catch up with the turtle, the argument is considered moot, no matter how logical it may appear (or may have appeared at the time the contra factum non valet argumentum principle was formulated).