2.20.1
The propositions which are true and evident are of necessity used even by those who contradict them: and a man might perhaps consider it to be the greatest proof of a thing being evident that it is found to be necessary even for him who denies it to make use of it at the same time. For instance, if a man should deny that there is anything universally true, it is plain that he must make the contradictory negation, that nothing is universally true. What, wretch, do you not admit even this? For what else is this than to affirm that whatever is universally affirmed is false? Again if a man should come forward and say: Know that there is nothing that can be known, but all things are incapable of sure evidence; or if another say, Believe me and you will be the better for it, that a man ought not to believe any thing; or again, if another should say, Learn from me, man, that it is not possible to learn any thing; I tell you this and will teach you, if you choose. Now in what respect do these differ from those? Whom shall I name? Those who call themselves Academics? 'Men, agree [with us] that no man agrees [with another]: believe us that no man believes anybody.'