Thomas Aquinas, Compendium of Theology

If the question were to be asked whether, in consequence of the removal of the personal properties by intellectual abstraction, the divine essence would remain, the answer is that in one respect it would remain, but in another it would not. Intellectual abstraction can take place in two ways. The first is by abstracting form from matter. In this abstraction the mind proceeds from the more formal to the more material; the first subject remains until the end, and the ultimate form is removed first. The second way of abstracting is by the abstraction of the universal from the particular, and this proceeds according to an order that is, in a sense, the opposite; the individuating material conditions are first removed, so that what is common may be retained.

In God, of course, there are neither matter and form, nor universal and particular. Nevertheless there is in the Godhead something that is common, and something that is proper and that supposes the common nature; for, in our human way of thinking, the divine persons are to the divine essence what individual supposita are to a common nature. According to the first type of intellectual abstraction, therefore, if we remove the personal properties, which are the subsisting persons themselves, the common nature does not remain. But in the second type of abstraction it does remain.