Aquinas preceded the existence of the discipline of epistemology, which began among modern thinkers whose positions, following in the wake of Descartes, are fundamentally opposed to Aquinas'. Nonetheless, Aquinas' thought is more measured than the crowd that requires epistemology. A Thomistic theory of knowledge can be derived from a mixture of Aquinas' logical, psychological, metaphysical, and even Theological doctrines. Aquinas' thought is an instance of the correspondence theory of truth, which says that something is true "when it conforms to the external reality." Therefore, any being that exists can be said to be true insofar that it participates in the world.