From his references, St Thomas appears to have been more familiar with the Timaeus than with any other of Plato's writings. That poetic, mystical and obscure dialogue was a special favourite of the Neoplatonists, from whom St Thomas gathered his knowledge of Plato. The passage, Timaeus, 69c-70a describing how "the mortal kind of soul," with its two divisions, was allocated in the body by inferior deities, after the Supreme Deity had produced the intellect, misled early commentators, and after them St Thomas, into the belief that Plato supposed three distinct souls in one human body. Plato never speaks of 'souls' except in reference to distinct bodies. He speaks of 'the soul' of man as familiarly as we do. The nous in the head, the thumos (St Thomas's pars irascibilis) in the chest, and the epithumiai (pars concupiscibilis) in the belly, are not three souls, but three varieties of one soul. Cf Timaeus, 89e, "three kinds of soul have been put to dwell in us in three several places: Tim, 79d "what the soul has of mortal and of divine in its being": Republic, 439e, "two kinds being in the soul": Rep, 441c, "there are varieties in the soul of each individual." In Laws, 863b, he doubts whether the phymos is to be called "an affection or a part of the soul." In the ultimate analysis of Plato's meaning nothing more will appear, I believe, than the triple division, accepted by Aristotle and St Thomas, of nous, phumos, epithumia, three phases of one soul, the first inorganic and spiritual, the two latter organic and involving connexion with the body.

Of God and His Creatures: 2.58