Theaetetus is delighted with this explanation. But Socrates has no sooner
found the new solution than he sinks into a fit of despondency. For an
objection occurs to him:--May there not be errors where there is no
confusion of mind and sense? e.g. in numbers. No one can confuse the man
whom he has in his thoughts with the horse which he has in his thoughts,
but he may err in the addition of five and seven. And observe that these
are purely mental conceptions. Thus we are involved once more in the
dilemma of saying, either that there is no such thing as false opinion, or
that a man knows what he does not know.

We are at our wit's end, and may therefore be excused for making a bold
diversion. All this time we have been repeating the words 'know,'
'understand,' yet we do not know what knowledge is. 'Why, Socrates, how
can you argue at all without using them?' Nay, but the true hero of
dialectic would have forbidden me to use them until I had explained them.
And I must explain them now. The verb 'to know' has two senses, to have
and to possess knowledge, and I distinguish 'having' from 'possessing.' A
man may possess a garment which he does not wear; or he may have wild birds
in an aviary; these in one sense he possesses, and in another he has none
of them. Let this aviary be an image of the mind, as the waxen block was;
when we are young, the aviary is empty; after a time the birds are put in;
for under this figure we may describe different forms of knowledge;--there
are some of them in groups, and some single, which are flying about
everywhere; and let us suppose a hunt after the science of odd and even, or
some other science. The possession of the birds is clearly not the same as
the having them in the hand. And the original chase of them is not the
same as taking them in the hand when they are already caged.

This distinction between use and possession saves us from the absurdity of