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 opinion consists in saying to
yourself, that one thing is another. But did you ever say to yourself,
that good is evil, or evil good? Even in sleep, did you ever imagine
that odd was even? Or did any man in his senses ever fancy that an ox
was a horse, or that two are one? So that we can never think one thing
to be another; for you must not meet me with the verbal quibble that
one--eteron--is other--eteron (both 'one' and 'oth