be true at any given instant. But the reply is in the end shown to
be inconsistent with the Heraclitean foundation, on which the doctrine
has been affirmed to rest. For if the Heraclitean flux is extended to
every sort of change in every instant of time, how can any thought
or word be detained even for an instant? Sensible perception, like
everything else, is tumbling to pieces. Nor can Protagoras himself
maintain that one man is as good as another in his knowledge of the
future; and 'the 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
relativ