Sokrates turns to another argument. If knowledge be the same thing as sensible perception, then it follows, that so soon as a man ceases to see and hear, he also ceases to know. The memory of what he has seen or heard, upon that supposition, is not knowledge. But Theætêtus admits that a man who remembers what he has seen or heard does know it. Accordingly, the answer that knowledge is sensible perception, cannot be maintained.[88] [Footnote 88: Plato, Theætêt. pp. 163, 164.] Here Sokrates makes out a good case against the answer in its present wording. But we may fairly doubt whether those who affirmed the matter of knowledge to consist in the facts of sense, ever meant to exclude memory. They meant probably the facts of sense both as perceived and as remembered; though the wording cited by Plato does not strictly include so much. Besides, we must recollect, that Plato includes in the meaning of the word Knowledge or Cognition an idea of perfect infallibility: distinguishing it generically from the highest form of opinion. But memory is a fallible process: sometimes quite trustworthy--under other circumstances, not so. Accordingly, memory, in a general sense, cannot be put on a level with present perception, nor said to generate what Plato calls knowledge. [Side-note: Argument from the analogy of seeing and not seeing at the same time .] The next argument of Plato is as follows. You can see, and not see, the same thing at the same time: for you may close one of your eyes, and look only with the other. But it is impossible to know a thing, and not to know it at the same time. Therefore _to know_ is not the same as _to see_.[89] [Footnote 89: Plato, Theætêt. p. 165 B.] This argument is proclaimed by Plato as a terrible puzzle, leaving no escape.[90] Perhaps he meant to speak ironically. In reality, this puzzle is nothing but a false inference deduced from a false premiss. The inference is false, because if we grant the premiss, that it is possible both to _see_ a thing, and _not to see_ it, at the same time--there is no reason why it should not also be possible to _know_ a thing, and _not to know_ it, at the same time. Moreover, the premiss is also false in the ordinary sense which the words bear: and not merely false, but logically impossible, as a sin against the maxim of contradiction. Plato procures it from a true premiss, by omitting an essential qualification. I see an object with my open eye: I do not see it with my closed eye. From this double proposition, alike intelligible and true, Plato thinks himself authorised to discard the qualification, and to tell me that I see a thing and do not see it--passing _à dicto secundum quid ad dictum simpliciter_. This is the same liberty which he took with the Protagorean doctrine. Protagoras having said--"Every thing which any man believes is true _to that man_"--Plato reasons against him as if he had said--"Every thing which any man believes is _true_".