Action is self-determined in all animals: judgement on the propriety of action is self-determined in man alone among animals. That self-determination of judgement means free will. Free will is due to the power of forming universal ideas, or general concepts, of the suitable and the good (or to what Plato might have called the vision of the idea of the good -- Rep. VI, 505: cf. Phaedrus, 248, 249). Such is the momentous teaching of St Thomas in this chapter.
Of God and His Creatures: 2.48