Theaetetus | Page 7

expedient,' if not 'the just and true,' belongs to the sphere of the future.
And so we must ask again, What is knowledge? The comparison of
sensations with one another implies a principle which is above
sensation, and which resides in the mind itself. We are thus led to look
for knowledge in a higher sphere, and accordingly Theaetetus, when
again interrogated, replies (2) that 'knowledge is true opinion.' But how
is false opinion possible? The Megarian or Eristic spirit within us
revives the question, which has been already asked and indirectly
answered in the Meno: 'How can a man be ignorant of that which he
knows?' No answer is given to this not unanswerable question. The
comparison of the mind to a block of wax, or to a decoy of birds, is
found wanting.
But are we not inverting the natural order in looking for opinion before
we have found knowledge? And knowledge is not true opinion; for the
Athenian dicasts have true opinion but not knowledge. What then is
knowledge? We answer (3), 'True opinion, with definition or
explanation.' But all the different ways in which this statement may be
understood are set aside, like the definitions of courage in the Laches,
or of friendship in the Lysis, or of temperance in the Charmides. At
length we arrive at the conclusion, in which nothing is concluded.
There are two special difficulties which beset the student of the
Theaetetus: (1) he is uncertain how far he can trust Plato's account of
the theory of Protagoras; and he is also uncertain (2) how far, and in
what parts of the dialogue, Plato is expressing his own opinion. The
dramatic character of the work renders the answer to both these
questions difficult.
1. In reply to the first, we have only probabilities to offer. Three main
points have to be decided: (a) Would Protagoras have identified his
own thesis, 'Man is the measure of all things,' with the other, 'All
knowledge is sensible perception'? (b) Would he have based the
relativity of knowledge on the Heraclitean flux? (c) Would he have
asserted the absoluteness of sensation at each instant? Of the work of
Protagoras on 'Truth' we know nothing, with the exception of the two

famous fragments, which are cited in this dialogue, 'Man is the measure
of all things,' and, 'Whether there are gods or not, I cannot tell.' Nor
have we any other trustworthy evidence of the tenets of Protagoras, or
of the sense in which his words are used. For later writers, including
Aristotle in his Metaphysics, have mixed up the Protagoras of Plato, as
they have the Socrates of Plato, with the real person.
Returning then to the Theaetetus, as the only possible source from
which an answer to these questions can be obtained, we may remark,
that Plato had 'The Truth' of Protagoras before him, and frequently
refers to the book. He seems to say expressly, that in this work the
doctrine of the Heraclitean flux was not to be found; 'he told the real
truth' (not in the book, which is so entitled, but) 'privately to his
disciples,'--words which imply that the connexion between the
doctrines of Protagoras and Heracleitus was not generally recognized in
Greece, but was really discovered or invented by Plato. On the other
hand, the doctrine that 'Man is the measure of all things,' is expressly
identified by Socrates with the other statement, that 'What appears to
each man is to him;' and a reference is made to the books in which the
statement occurs;--this Theaetetus, who has 'often read the books,' is
supposed to acknowledge (so Cratylus). And Protagoras, in the speech
attributed to him, never says that he has been misunderstood: he rather
seems to imply that the absoluteness of sensation at each instant was to
be found in his words. He is only indignant at the 'reductio ad
absurdum' devised by Socrates for his 'homo mensura,' which
Theodorus also considers to be 'really too bad.'
The question may be raised, how far Plato in the Theaetetus could have
misrepresented Protagoras without violating the laws of dramatic
probability. Could he have pretended to cite from a well-known writing
what was not to be found there? But such a shadowy enquiry is not
worth pursuing further. We need only remember that in the criticism
which follows of the thesis of Protagoras, we are criticizing the
Protagoras of Plato, and not attempting to draw a precise line between
his real sentiments and those which Plato has attributed to him.
2. The other difficulty is a more subtle, and also a more important one,
because bearing on the general character of the Platonic dialogues. On
a first reading of them, we are apt to imagine that the truth is only
spoken by Socrates,