V. Having rejected the doctrine that 'Knowledge is perception,' we now
proceed to look for a definition of knowledge in the sphere of opinion.
But here we are met by a singular difficulty: How is false opinion
possible? For we must either know or not know that which is presented to
the mind or to sense. We of course should answer at once: 'No; the
alternative is not necessary, for there may be degrees of knowledge; and we
may know and have forgotten, or we may be learning, or we may have a
general but not a particular knowledge, or we may know but not be able to
explain;' and many other ways may be imagined in which we know and do not
know at the same time. But these answers belong to a later stage of
metaphysical discussion; whereas the difficulty in question naturally
arises owing to the childhood of the human mind, like the parallel
difficulty respecting Not-being. Men had only recently arrived at the
notion of opinion; they could not at once define the true and pass beyond
into the false. The very word doxa was full of ambiguity, being sometimes,
as in the Eleatic philosophy, applied to the sensible world, and again used
in the more ordinary sense of opinion. There is no connexion between
sensible appearance and probability, and yet both of them met in the word
doxa, and could hardly be disengaged from one another in the mind of the
Greek living in the fifth or fourth century B.C. To this was often added,
as at the end of the fifth book of the Republic, the idea of relation,
which is equally distinct from either of them; also a fourth notion, the
conclusion of the dialectical process, the making up of the mind after she
has been 'talking to herself' (Theat.).

We are not then surprised that the sphere of opinion and of Not-being
should be a dusky, half-lighted place (Republic), belonging neither to the
old world of sense and imagination, nor to the new world of reflection and
reason. Plato attempts to clear up this darkness. In his accustomed