mpression of
them as from the seal of a ring; and that we remember and know what is
imprinted as long as the image lasts; but when the image is effaced, or
cannot be taken, then we forget and do not know.

THEAETETUS: Very good.

SOCRATES: Now, when a person has this knowledge, and is considering
something which he sees or hears, may not false opinion arise in the
following manner?

THEAETETUS: In what manner?

SOCRATES: When he thinks what he knows, sometimes to be what he knows,
and sometimes to be what he does not know. We were wrong before in
denying the possibility of this.

THEAETETUS: And how would you amend the former statement?

SOCRATES: I should begin by making a list of the impossible cases which
must be excluded. (1) No one can think one thing to be another when he
does not perceive either of them, but has the memorial or seal of both
of them in his mind; nor can any mistaking of one thing for another
occur, when he only knows one, and does not know, and has no impression
of the other; nor can he think that one thing which he does not know is
another thing which he does not know, or that what he does not know
is what he knows; nor (2) that one thing which he perceives is another
thing which he perceives, or that something which he perceives is
something which he does not perceive; or that something which he does
not perceive is something else which he does not perceive; or that
something which he does not perceive is something which he perceives;
nor again (3) can he think that something which he knows and perceives,
and of which he has the impression coinciding with sense, is something
else which he knows and perceives, and of which he has the impression
coinciding with sense;--this last case, if possible, is still more
inconceivable than the others; nor (4) can he think that something which
he knows and perceives, and of which he has the memorial coinciding with
sense, is something else which he knows; nor so long as these agree,
can he think that a thing which he knows and perceives is another thing
which he perceives; or that a thing which he does not know and does not
perceive, is the same as another thing which he does not know and does
not perceive;--nor again, can he suppose that a thing which he does not
know and does not perceive is the same as another thing which he does
not know; or that a thing which he does not know and does not perceive
is another thing which he does not perceive:--All