Pleasure and Illusion in Plato

Plato links pleasure with illusion, and this link explains his rejection of the view that all desires are rational desires for the good. The Protagoras and Gorgias show connections between pleasure and illusion; the Republic develops these into a psychological theory. One part of the soul is not only prone to illusions, but also incapable of the kind of reasoning that can dispel them. Pleasure appears good; therefore this part of the soul (the appetitive part) desires pleasures qua good but ignores reasoning about what is really good. Hence the new moral psychology of the Republic, not all desires are rational, and thus virtue depends on bringing one's non-rational desires under the control of reason.