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withdrawn.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 are
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 judgements 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,