Excerpts from Books and Wikipedia
"He was convinced that ethical conduct must be founded on knowledge, and that that knowledge must be knowledge of eternal values which are not subject to the shifting and changing impressions of sense or of subjective opinion, but are the same for all men and for all peoples and all ages. . . . Sensible things are copies or participations in these universal realities, but the latter abide in an unchanging heaven of their own, while sensible things are subject to change, in fact are always becoming and can never truly be said to be. . . . Thus in the Phaedo he teaches that the soul existed before its union with the body in a transcendent realm, where it beheld the subsistent intelligible entities or Ideas, which would seem to constitute a plurality of 'detached' essences. The process of knowledge, or getting to know, consists essentially in recollection, in remembering the Ideas which the soul once beheld clearly in its state of pre-existence." [Copleston: History of Philosophy I-III, p. 143, 166]