Critolaus seems to have paid particular attention to rhetoric, though he considered it, like Aristotle, not as an art, but rather as a matter of practice. Cicero speaks in high terms of his eloquence. Next to Rhetoric, Critolaus seems to have given his chief attention to the study of moral philosophy, and to have made some additions to Aristotle's system. In general, he deviated very little from the philosophy of the founder of the Peripatetic school, though in some respects he went beyond his predecessors. For example, he held that pleasure is an evil, and definitely maintained that the soul consists of aether. The end of existence was to him the general perfection of the natural life, including the goods of the soul and the body, and also external goods. Cicero says in the "Tusculanae Quaestiones" that the goods of the soul entirely outweighed for him the other goods ().