Thomas Aquinas, Compendium of Theology

The view proposed by Gilbert de la Porree and some of his followers, that the properties under discussion are not in the persons, but are external to them, cannot be defended. Real relations must be in the things that are related. This is evident in the case of creatures, for real relations are in them as accidents in their subjects. But the relations whereby the persons are distinguished within the Godhead are real relations, as was demonstrated above. Hence they must be in the divine persons; but not, of course, as accidents. Other perfections, too, which in creatures are accidents, cease to be accidents when transferred to God, as was shown above; such are wisdom, justice, and the like.

Besides, there can be no distinction in God except through the relations; all perfections that are predicated absolutely are common. Therefore, if the relations were external to the persons, no distinction would remain among the persons themselves. And so there are relative properties in the persons; but they are the persons themselves, and also the divine essence itself. In the same way wisdom and goodness are said to be in God, and are God Himself, as well as the divine essence, as was said above.