Commentaria in Summam contra Gentiles, Liber IV

After St. Thomas has shown from the authorities of Sacred Scripture that generation, filiation, and paternity are in the divine, he wishes now to investigate the manner of the said generation¹. Concerning this, however, he does three things: first, he introduces arguments against that generation; second, he determines the truth, in the following chapter; third, he answers the objections, in chapter 14.

As to the first, he says that, since it is proposed to us to be believed from the Sacred Scriptures that the Father and the Son, even if they are distinguished in persons, are nevertheless one God and have one essence or nature; and since this seems very remote from the nature of creatures: human reason, proceeding from the properties of creatures, suffers manifold difficulty in this mystery².

And first, because generation known to us is a kind of change, to which corruption is opposed. But God is immutable, incorruptible, and eternal.

Second, because it follows, if the Son of God is begotten, that he is neither eternal—as coming forth from potency into act, since everything that is generated is mutable, and whatever is changed comes forth from potency into act—nor true God, because he has something of potentiality.

Third, because the Son received from the Father neither another nature numerically, but the same specifically, since there cannot be several godheads numerically; nor the same numerically, because he received neither a part of it, since the divine nature is indivisible; nor the whole of it, because thus it would cease to be in the Father, as being poured into the Son, and thus the Father by generating would be corrupted; nor can this be through a kind of exuberance, as the water of a spring flows into a stream and the spring is not emptied, because the divine nature, just as it cannot be divided, so neither can it be increased. But if he received a nature of another genus from the Father, as happens in equivocal generations, it follows that the Son is not true God.

Fourth, because it follows that the Son is not his own essence or nature: because then it would be necessary that the recipient be one thing, and the nature received another.

Fifth, because, if the Son is not other than his essence, since the divine essence is subsisting; and the Father also is his essence; it remains that the Father and the Son agree in the same subsisting thing; and consequently that they agree in person, since in intellectual natures a subsisting thing is called a person. But if the Son is not the divine essence itself, he will not be true God.

Sixth, because, since in the Father there is no other principle of individuation than his essence, from the fact that the Father is the same as God, while the essence of God is that by which God is individuated, as not being a form in matter, his essence can be in no other supposit: for that which is the principle of individuation in each thing, it is impossible to belong to another supposit. And thus it follows either that it is not in the Son, and thus he will not be true God; or that the Son is not distinct in supposit from the Father.

Seventh, because, if the Father and the Son are two supposits and one in essence, there must be in them something besides the essence by which they are distinguished. And thus the person of the Son will be composite, and likewise the person of the Father: and thus neither will be true God.

If it be said that they are distinguished by relation alone, inasmuch as one is the Father and the other is the Son; but those things which are said relatively do not predicate something else³ but toward something else:—this does not seem sufficient. First, because, since a relation cannot exist without something absolute, either that absolute in which the relation of the Father and the Son is founded is only one, or two. Not the first: because in the same thing a twofold relation cannot be founded, unless perhaps a relation of identity, which cannot work a distinction. Therefore the second. Therefore it is necessary that a distinction of absolutes be pre-understood. Therefore etc.—Second, because the relation that distinguishes the Son from the Father, if it is a real thing, since it does not seem to be the divine essence itself, because that is common, there will be in the Son some real thing that is not his essence: and thus he will not be true God. But if it is in the intellect alone, it will not be able personally to distinguish the Son from the Father.—Third, because then neither will be true God. For every relative depends on its correlative: but what depends on another is not true God.

Eighth, because, if the Father is God and likewise the Son, this name God will be predicated substantially of both. But it is impossible that plurality be found on the part of the subjects, when there is unity on the part of the substantial predicate (understand this of numerical unity): because a substantial predicate is that which truly is that very thing of which it is predicated. And thus it seems, if the Father and the Son are two persons, to be impossible that they are one God.

Ninth, because opposite things are predicated of God the Father and of God the Son: for the Father is unbegotten God, but the Son is begotten God. But opposite predicates show plurality in that of which they are predicated.

Lastly, for the solution that follows, St. Thomas adds that, because the truth is strong in itself and is shattered by no attack, it must be shown that the truth of faith cannot be overcome by reason.