Theaetetus

SOCRATES: It must be stated in this way about them, distinguishing from the beginning, that for one to think that what he knows—having the memory of it in his soul, but not perceiving it—is some other thing of the things he knows—having also the imprint of that, but not perceiving it—is impossible. And again, to think that what he knows is what he does not know and of which he has no seal; and what he does not know, to be what he does not know; and what he does not know, to be what he knows; and what he perceives, to think it is some other thing of the things he perceives; and what he perceives, to be some of the things he does not perceive; and what he does not perceive, to be some of the things he does not perceive; and what he does not perceive, to be some of the things he perceives. And still further, again, to think that what he knows and perceives, and of which he holds the sign corresponding to the perception, is some other thing of the things he knows and perceives, and of which he holds in turn the sign of that other corresponding to the perception, is even more impossible than those, if that were possible. And to think that what he knows and perceives, having the memory correctly, is what he knows is impossible; and what he knows and perceives, having it in the same way, to be what he perceives; and again, what he does not know nor perceive, to be what he does not know nor perceive; and what he does not know nor perceive, to be what he does not know; and what he does not know nor perceive, to be what he does not perceive. All these cases exceed in the impossibility of anyone opining falsely in them. It remains, then, in the following cases, if indeed anywhere else, for such a thing to occur. THEAETETUS: In which, indeed? If perhaps I might learn something more from them; for now I am not following. SOCRATES: In those things which he knows, to think that they are some other things of those he knows and perceives; or of those which he does not know, but perceives; or of those which he knows and perceives, to be some of those he knows in turn and perceives. THEAETETUS: Now I am left behind much more than before. SOCRATES: Listen, then, back again in this way. Do I, knowing Theodorus and remembering in myself what sort of person he is, and Theaetetus in the same way, sometimes indeed see them, and sometimes not, and sometimes touch them, and sometimes not, and hear them or perceive some other perception, and at other times have indeed no perception about you, but remember you none the less and know you in myself? THEAETETUS: Certainly indeed. SOCRATES: Learn this first, then, of what I wish to show: that it is possible for one not to perceive what he knows, and it is possible to perceive it. THEAETETUS: True. SOCRATES: Is it not also the case that what he does not know, it is often possible not even to perceive, but often possible only to perceive? THEAETETUS: This is also so.