7 In the first place, you yourselves, too, see clearly that, if you ever discuss obscure subjects, and seek to lay bare the mysteries of nature, on the one hand you do not know the very things which you speak of, which you affirm, which you uphold very often with special zeal, and that each one defends with obstinate resistance his own suppositions as though they were proved and ascertained truths. For how can we of ourselves know whether we perceive the truth, even if all ages be employed in seeking out knowledge— we whom some envious power brought forth, and formed so ignorant and proud, that, although we know nothing at all, we yet deceive ourselves, and are uplifted by pride and arrogance so as to suppose ourselves possessed of knowledge?
For, to pass by divine things, and those plunged in natural obscurity, can any man explain that which in the Phaedrus the well-known Socrates cannot comprehend— what man is, or whence he is, uncertain, changeable, deceitful, manifold, of many kinds? For what purposes he was produced? By whose ingenuity he was devised? What he does in the world? (C) why he undergoes such countless ills? Whether the earth gave life to him as to worms and mice, being affected with decay through the action of some moisture; or whether he received these outlines of body, and this cast of face, from the hand of some maker and framer?
Can he, I say, know these things, which lie open to all, and are recognisable by the senses common to all,— by what causes we are plunged into sleep, by what we awake? In what ways dreams are produced, in what they are seen? Nay rather— as to which Plato in the Theoetetus is in doubt— whether we are ever awake, or whether that very state which is called waking is part of an unbroken slumber? And what we seem to do when we say that we see a dream? Whether we see by means of rays of light proceeding towards the object, or images of the objects fly to and alight on the pupils of our eyes?
Whether the flavour is in the things tasted, or arises from their touching the palate? From what causes hairs lay aside their natural darkness, and do not become gray all at once, but by adding little by little? Why it is that all fluids, on mingling, form one whole; that oil, on the contrary, does not suffer the others to be poured into it, but is ever brought together clearly into its own impenetrable substance? Finally, why the soul also, which is said by you to be immortal and divine, is sick in men who are sick, senseless in children, worn out in doting, silly, and crazy old age?
Now the weakness and wretched ignorance of these theories is greater on this account, that while it may happen that we at times say something which is true, we cannot be sure even of this very thing, whether we have spoken the truth at all.
Source: Against the Heathen (New Advent)