“The human soul is considered by Plato to be an immaterial agent, superior in nature to the body and somewhat hindered by the body in the performance of the higher, psychic functions of human life. The tripartite division of the soul becomes an essential teaching of Platonic psychology from the Republic onward. The rational part is highest and is pictured as the ruler of the psychological organism in the well-regulated man.
“Next in importance is the “spirited” element of the soul, which is the source of action and the seat of the virtue of courage. The lowest part is the concupiscent or acquisitive element, which may be brought under control by the virtue of temperance The latter two are often combined and called irrational in contrast to the highest part.
“Sensation is an active function of the soul, by which the soul “feels” the objects of sense through the instrumentality of the body. Particularly in the young, sensation is a necessary prelude to the knowledge of Ideas, but the mature and developed soul must learn to rise above sense perception and must strive for a more direct intuition of intelligible essences.
“That the soul exists before the body (related to the Pythagorean and, possibly, Orphic doctrine of transmigration) and knows the world of Ideas immediately in this anterior condition, is the foundation of the Platonic theory of reminiscence (Meno, Phaedo, Republic, Phaedrus). Thus the soul is born with true knowledge in it, but the soul, due to the encrustation of bodily cares and interests, cannot easily recall the truths innately, and we might say now, subconsciously present in it.
“Sometimes sense perceptions aid the soul in the process of reminiscence, and again, as in the famous demonstration of the Pythagorean theorem by the slave boy of the Meno, the questions and suggestions of a teacher provide the necessary stimuli for recollection. The personal immortality of the soul is very clearly taught by Plato in the tale of Er (Repub. X) and, with various attempts at logical demonstration, in the Phaedo. Empirical and physiological psychology is not stressed in Platonism, but there is an approach to it in the descriptions of sense organs and their media in the Timaeus 42 ff.”
— Vernon J. Bourke, “Platonism,” in Dictionary of Philosophy, ed. by Dagobert D. Runes