24 Why, O Plato, do you in the Meno put to a young slave certain questions relating to the doctrines of number, and strive to prove by his answers that what we learn we do not learn, but that we merely call back to memory those things which we knew in former times? Now, if he answers you correctly—for it would not be becoming that we should refuse credit to what you say—he is led to do so not by his real knowledge, but by his intelligence; and it results from his having some acquaintance with numbers, through using them every day, that when questioned he follows your meaning, and that the very process of multiplication always prompts him.
But if you are really assured that the souls of men are immortal and endowed with knowledge when they fly hither, cease to question that youth whom you see to be ignorant and accustomed to the ways of men; call to you that man of forty years, and ask of him, not anything out of the way or obscure about triangles, about squares, not what a cube is, or a second power, the ratio of nine to eight, or finally, of four to three; but ask him that with which all are acquainted— what twice two are, or twice three.
We wish to see, we wish to know, what answer he gives when questioned— whether he solves the desired problem. In such a case will he perceive, although his ears are open, whether you are saying anything, or asking anything, or requiring some answer from him? And will he not stand like a stock, or the Marpesian rock, as the saying is, dumb and speechless, not understanding or knowing even this— whether you are talking with him or with another, conversing with another or with him; whether that is intelligible speech which you utter, or merely a cry having no meaning, but drawn out and protracted to no purpose?
Source: Against the Heathen (New Advent)