The first argument is that all things which have opposites are generated from their opposites--a
statement which reminds us of Anaximander's views on cosmic justice. Now life and death are
opposites, and therefore each must generate the other. It follows that the souls of the dead exist
somewhere, and come back to earth in due course. Saint Paul's statement, "the seed is not
quickened except it die," seems to belong to some such theory as this.
The second argument is that knowledge is recollection, and therefore the soul must have existed
before birth. The theory that knowledge is recollection is supported chiefly by the fact that we
have ideas, such as exact equality, which cannot be derived from experience. We have experience
of approximate equality, but absolute equality is never found among sensible objects, and yet we
know what we mean by "absolute equality." Since we have not learnt this from experience, we
must have brought the knowledge with us from a previous existence. A similar argument, he says,
applies to all other ideas. Thus the existence of essences, and our capacity to apprehend them,
proves the pre-existence of the soul with knowledge.
The contention that all knowledge is reminiscence is developed at greater length in the Meno (87
ff.). Here Socrates says "there is no teaching, but only recollection." He professes to prove his
point by having Meno call in a slave-boy whom Socrates proceeds to question on geometrical
problems. The boy's answers are supposed to show that he really knows geometry, although he has
hitherto been unaware of possessing this knowledge. The same conclusion is drawn in the Meno
as in the Phaedo, that knowledge is brought by the soul from a previous existence.
As to this, one may observe, in the first place, that the argument is wholly inapplicable to
empirical knowledge. The slave-boy could not have been led to "remember" when the Pyramids
were built, or whether the siege of Troy really occurred, unless he had happened to be present at
these events. Only the sort of knowledge that is called a priori--especially logic and mathematics--
can be possibly supposed to exist in every one independently of experience. In fact, this is the only
sort of knowledge (apart from mystic insight) that Plato admits to be really knowledge. Let us see
how the argument can be met in regard to mathematics.