As a result of his exaggerated intellectualism he [Plato] failed to distinguish the acts of the practical from those of the speculative intellect and identified virtue, which requires rectitude of the will, with knowledge, which is a perfection of the reason alone.
He therefore misapplied the principle, in itself true, that the will always follows the guidance of the understanding, and maintained that sin is simply due to lack of knowledge and that no one deliberately does evil: "the sinner is merely an ignorant person."
The consequence of this theory, which Plato did not intend, is the denial of free will.
(Jacques Maritain, Introduction to Philosophy)
