Therefore, God wished to manifest His infinite mercy in such a way that He would in no way derogate from His justice. This was done while He became man for us, so that He might make satisfaction for us, in which even more abundant mercy was shown to us than if He had dismissed sin without satisfaction, insofar as He exalted our nature more and suffered death for us. To the third, it is responded by denying that original sin is a lesser evil. For this, it must be noted that the quantity of original and actual mortal sin can be considered in two ways: either according to the principle or according to the good which is deprived by either. The principle of actual sin is one's own will. The principle of original [sin] in this one is the vitiated origin of him. Whence, original [sin] is in a way necessary, but actual [sin] is entirely voluntary; whence it has more of the nature of guilt and is more blameworthy. But the good which is corrupted by actual sin is the good of this person, to which the good of the whole nature, which is corrupted by original [sin], is preferred; because the good of the people is more divine than the good of one man, as is said in Book II of the Ethics Aristotle's Nicomachean Ethics. So it can be said that actual [sin] is more of a fault, but original [sin] is more of an evil. The action of the one making satisfaction is proportioned to the quantity of the guilt on the part of the good which is corrupted by the guilt. And therefore, since every action is of a person (because acts belong to individuals), therefore, for satisfaction for actual [sin], the act of any man with divine grace suffices, but not for satisfaction for original sin, unless the action of that man were worth more than the whole good of human nature; and thus it could not be a mere man, and therefore it behooved that there be a God and man who would make satisfaction for original [sin].

To the second question, it is proceeded thus: And it seems that even if man had not sinned, God would have been incarnate. First, thus: Deuteronomy xxxii, "God's works are perfect," but perfection cannot be unless the last is joined to the first principle, so that as it were a certain circle is closed and no addition could be made. Since, therefore, God Himself is the beginning and man is the last of creatures, it was fitting for the perfection of the universe that, even if man had not sinned, God would become man. Second, thus: Romans i, "The invisible things of God are understood, being perceived by the things that are made," but the power, wisdom, and goodness of God are infinite; therefore, it was fitting, even if man had not sinned, that these be manifested in some effect; but infinite power is not manifested except through an infinite effect, nor infinite wisdom except through infinite beauty, nor infinite goodness except through infinite communication. Since, therefore, no creature is infinite, nor is there infinite beauty in it from form and proportion of parts, nor again does any creature receive an infinite communication of good, it seems that it was fitting, even if man had not sinned, for God to be united to man so that on the part of man there would be reason and on the part of God infinity, and from the conjunction of God to the creature, infinite beauty would resplend and an infinite good would be communicated to human nature itself, namely the uncreated person who would subsist in it. Third, thus: The capacity of human nature is not expanded by sin in any way; but after sin, human nature was found to be capable of such a great good that it would be assumed by God in the unity of person; therefore, even before sin, it was capable of such dignity; but it pertains to God, who loves with infinite love those things that are, to deny no good to the creature of which it is capable; therefore, He would have assumed human nature even if man had not sinned.