Discourses: Book 2 20 : 1
The propositions which are true and evident must of necessity be employed even by those who contradict them; and one might consider as perhaps the strongest proof of a proposition being evident the fact that even the man who contradicts it finds himself obliged at the same time to employ it. For example, if a man should contradict the proposition that there is a universal statement which is true, it is clear that he must assert the contrary, and say: No universal statement is true. Slave, this is not true, either. For what else does this assertion amount to than: If a statement is universal, it is false? Again, if a man comes forward and says, "I would have you know that nothing is knowable, but that everything is uncertain"; or if someone else says, "Believe me, and it will be to your advantage, when I say: One ought not to believe a man at all"; or again, someone else, "Learn from me, man, that it is impossible to learn anything; it is I who tell you this and I will prove it to you, if you wish," what difference is there between these persons and—whom shall I say?—those who call themselves Academics? "O men," say the Academics, "give your assent to the statement that no man assents to any statement; believe us when we say that no man can believe anybody."