Commentaria in Primam Partem Summae Theologiae

In the title, proper is distinguished against common. – In the body, two conclusions. The first is: Eternity truly and properly exists in God alone. It is proved. God alone is entirely immutable; therefore God alone is truly and properly eternal. – The inference holds: because eternity follows upon immutability.

The second is: Eternity is found participatively in other things. It is proved. Immutability is participated in four ways by other things; therefore, eternity. – The inference holds: because insofar as some things are immutable, to that extent they are eternal. The antecedent is explained according to the four degrees of immutability. The first is according to long duration: and in this respect things are corruptible but very long-lived, such as mountains and hills. The second is according to perpetuity with respect to the whole: and in this respect the elements are corruptible with respect to their parts but not with respect to the whole—although in the text these degrees are enumerated in an inverted order. The third is according to substantial incorruptibility simply speaking: and in this respect are heavenly bodies and angels. The fourth is according to the immutability of operation, entirely of one and the same number: and in this respect are the blessed as such.

II. In response to the third, if you do not know, novice, how it is to be understood that "necessary" signifies a certain mode of truth, recall book two of Perihermenias¹, where the modal propositions are treated; and you will see that "necessary," in its signified act, adds above the proposition a mode of composition, in which alone truth is found. And thus you will understand that "necessary," in its exercised act, of which this argument makes mention, truly exercises the mode of composition, by reason of which both the principles and the conclusions of demonstrations are called necessary.