The Refutation of All Heresies: Book I. 22The Academics; Difference of Opinion Among Them.

And another opinion of the philosophers was called that of the Academics, on account of those holding their discussions in the Academy, of whom the founder Pyrrho, from whom they were called Pyrrhonean philosophers, first introduced the notion of the incomprehensibility of all things, so as to (be ready to) attempt an argument on either side of a question, but not to assert anything for certain; for that there is nothing of things intelligible or sensible true, but that they appear to men to be so; and that all substance is in a state of flux and change, and never continues in the same (condition). Some followers, then, of the Academics say that one ought not to declare an opinion on the principle of anything, but simply making the attempt to give it up; whereas others subjoined the formulary "not rather" (this than that), saying that the fire is not rather fire than anything else. But they did not declare what this is, but what sort it is.