Thomas Aquinas

“ The sixth is the abstraction of a species from matter, as sense receives the species from the sensible object; wherein is wanting equality of spiritual simplicity. The seventh is the exciting of the will by knowledge, which excitation is merely temporal. The eighth is transformation, as an image is made of brass; which transformation is material. The ninth is motion from a mover; and here again we have effect and cause. The tenth is the taking of species from genera; but this mode has no place in God, for the Father is not predicated of the Son as the genus of a species. ”

“ Now the reason of this is that unequal things cannot have the same quantity. But quantity, in God, is nothing else than His essence. Wherefore it follows, that if there were any inequality in the divine persons, they would not have the same essence; and thus the three persons would not be one God; which is impossible. We must therefore admit equality among the divine persons. ”

“ And so as Augustine (Fulgentius, De Fide ad Petrum i) says: "We understand equality to be in the Father, Son and Holy Ghost, inasmuch as no one of them either precedes in eternity, or excels in greatness, or surpasses in power."
Reply to Objection 2: Where we have equality in respect of virtual quantity, equality includes likeness and something besides, because it excludes excess. For whatever things have a common form may be said to be alike, even if they do not participate in that form equally, just as the air may be said to be like fire in heat ”

“ Equality is measured by greatness. In God greatness signifies the perfection of nature, as above explained (A [1] , ad 1) , and belongs to the essence. Thus equality and likeness in God have reference to the essence; nor can there be inequality or dissimilitude arising from the distinction of the relations. ”

“ In God relation is not a universal whole, although it is predicated of each of the relations; because all the relations are one in essence and being, which is irreconcilable with the idea of universal, the parts of which are distinguished in being. Persons likewise is not a universal term in God as we have seen above (Q [30] , A [4] ) . Wherefore all the relations together are not greater than only one; nor are all the persons something greater than only one; because the whole perfection of the divine nature exists in each person. ”

“ In natural agents, however, the same happens from the agent not having its perfection of natural power from the very first, but obtaining it after a certain time; as, for instance, a man is not able to generate from the very first. Considered on the part of action, anything derived from a principle cannot exist simultaneously with its principle when the action is successive. ”

“ The second effect is operation, for every agent acts through its form. Consequently virtual quantity is measured both in regard to being and in regard to action: in regard to being, forasmuch as things of a more perfect nature are of longer duration; and in regard to action, forasmuch as things of a more perfect nature are more powerful to act. ”

“ It would seem that the Son and the Father are not in each other. For the Philosopher (Phys. iv, text. 23) gives eight modes of one thing existing in another, according to none of which is the Son in the Father, or conversely; as is patent to anyone who examines each mode. Therefore the Son and the Father are not in each other.
Objection 2: Further, nothing that has come out from another is within. But the Son from eternity came out from the Father, according to Mic. 5:2: "His going forth is from the beginning, from the days of eternity." Therefore the Son is not in the Father. ”

“ Eternity excludes the principle of duration, but not the principle of origin.
Reply to Objection 3: Every corruption is a change; and so all that corrupts begins not to exist and ceases to be. The divine generation, however, is not changed, as stated above (Q [27] , A [2] ) . Hence the Son is ever being begotten, and the Father is always begetting. ”

“ The eleventh is the realization of an idea [ideatio] , as an external coffer arises from the one in the mind. The twelfth is birth, as a man is begotten of his father; which implies priority and posteriority of time. ”

“ Yet, above them all the procession of the word from the intellect represents it more exactly; the intellectual word not being posterior to its source except in an intellect passing from potentiality to act ”