(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,