In his doctrine of the Trinity, Aquinas starts from the Augustinian system. Since God has only the functions of thinking and willing, only two "processiones" can be asserted from the Father; but these establish definite relations of the persons of the Trinity, one to another. The relations must be conceived as real and not as merely ideal; for, as with creatures relations arise through certain accidents, since in God there is no accident but all is substance, it follows that "the relation really existing in God is the same as the essence according to the thing". From another side, however, the relations as real must be really distinguished one from another. Therefore, three persons are to be affirmed in God.