Freud's basic tripartite psychodynamic model of the psyche with the Id contains unconscious libido and aggressive drives as well as conscious Ego and Superego faculties. Although having, prima facie, a very similar architecture as Plato's model of the psyche, Freud's treatment effectively inverts the former's emphasis on the power of reason (in this he is inspired by Schopenhauer). You will also recall, crudely put, that Freud postulated that the Id demands instant gratification; the moralistic conscientious Superego is against gratification and that the Ego attempts to reconcile the former's via delayed gratification. The Ego, as a "third term", tries to balance the antithetical demands of the "psychotic" Id and "neurotic" Superego. If it does not, the individual is considered to be mentally ill.