therefore fails of truth; and therefore has no share in knowledge. But if
so, knowledge is not perception. What then is knowledge? The mind, when
occupied by herself with being, is said to have opinion--shall we say that
'Knowledge is true opinion'? But still an old difficulty recurs; we ask
ourselves, 'How is false opinion possible?' This difficulty may be stated
as follows:--

Either we know or do not know a thing (for the intermediate processes of
learning and forgetting need not at present be considered); and in thinking
or having an opinion, we must either know or not know that which we think,
and we cannot know and be ignorant at the same time; we cannot confuse one
thing which we do not know, with another thing which we do not know; nor
can we think that which we do not know to be that which we know, or that
which we know to be that which we do not know. And what other case is
conceivable, upon the supposition that we either know or do not know all
things? Let us try another answer in the sphere of being: 'When a man
thinks, and thinks that which is not.' But would this hold in any parallel
case? Can a man see and see nothing? or hear and hear nothing? or touch
and touch nothing? Must he not see, hear, or touch some one existing
thing? For if he thinks about nothing he does not think, and not thinking
he cannot think falsely. And so the path of being is closed against us, as
well as the path of knowledge. But may there not be 'heterodoxy,' or
transference of opinion;--I mean, may not one thing be supposed to be
another? Theaetetus is confident that this must be 'the true falsehood,'
when a man puts good for evil or evil for good. Socrates will not
discourage him by attacking the paradoxical expression 'true falsehood,'
but passes on. The new notion involves a process of thinking about two
things, either together or alternately. And thinking is the conversing of
the mind with herself, which is carried on in question and answer, until
she no longer doubts, but determines and forms an opinion. And false