History of Western Philosophy - Bertrand Russell [90]
A large part of the argument that follows is concerned with the characterization of perception; when once this is completed, it does not take long to prove that such a thing as perception has turned out to be cannot be knowledge.
Socrates adds to the doctrine of Protagoras the doctrine of Heraclitus, that everything is always changing, i.e. that 'all the things we are pleased to say "are" really are in process of becoming'. Plato believes this to be true of objects of sense, but not of the objects of real knowledge. Throughout the dialogue, however, his positive doctrines remain in the background.
From the doctrine of Heraclitus, even if it be only applicable to objects of sense, together with the definition of knowledge as perception, it follows that knowledge is of what becomes, not of what is.
There are, at this point, some puzzles of a very elementary character. We are told that, since 6 is greater than 4 but less than 12, 6 is both great and small, which is a contradiction. Again, Socrates is now taller than Theaetetus, who is a youth not yet full grown; but in a few years Socrates will be shorter than Theaetetus. Therefore Socrates is both tall and short. The idea of a relational proposition seems to have puzzled Plato, as it did most of the great philosophers down to Hegel (inclusive). These puzzles, however, are not very germane to the argument, and may be ignored.
Returning to perception, it is regarded as due to an interaction between the object and the sense-organ, both of which, according to the doctrine of Heraclitus, are always changing, and both of which, in changing, change the percept. Socrates remarks that when he is well he finds wine sweet, but when ill, sour. Here it is a change in the percipient that causes the change in the percept.
Certain objections to the doctrine of Protagoras are advanced, and some of these are subsequently withdrawn. It is urged that Protagoras ought equally to have admitted pigs and baboons as measures of all things, since they also are percipients. Questions are raised as to the validity of perception in dreams and in madness. It is suggested that, if Protagoras is right, one man knows no more than another: not only is Protagoras as wise as the gods, but, what is more serious, he is no wiser than a fool. Further, if one man's judgments are as correct as another's, the people who judge that Protagoras is mistaken have the same reason to be thought right as he has.
Socrates undertakes to find an answer to many of these objections, putting himself, for the moment, in the place of Protagoras. As for dreams, the percepts are true as percepts. As for the argument about pigs and baboons, this is dismissed as vulgar abuse. As for the argument that, if each man is the measure of all things, one man is as wise as another, Socrates suggests, on behalf of Protagoras, a very interesting answer, namely that, while one judgment cannot be truer than another, it can be better, in the sense of having better consequences. This suggests pragmatism.1
This answer, however, though Socrates has invented it, does not satisfy him. He urges, for example, that when a doctor foretells the course of my illness, he actually knows more of my future than I do. And when men differ as to what it is wise for the State to decree, the issue shows that some men had a greater knowledge as to the future than others had. Thus we cannot escape the conclusion that a wise man is a better measure of things than a fool.
All these are objections to the doctrine that each man is the measure of all things, and only indirectly to the doctrine that 'knowledge' means 'perception', in so far as this doctrine leads to the other. There is, however, a direct argument, namely that memory must be allowed as well as perception.