Thomas Aquinas

“ But when the mind is the rule or measure of things, truth consists in the equation of the thing to the mind; just as the work of an artist is said to be true, when it is in accordance with his art.
Now as works of art are related to art, so are works of justice related to the law with which they accord. Therefore God's justice, which establishes things in the order conformable to the rule of His wisdom, which is the law of His justice, is suitably called truth. Thus we also in human affairs speak of the truth of justice. ”

“ Certain works are attributed to justice, and certain others to mercy, because in some justice appears more forcibly and in others mercy. Even in the damnation of the reprobate mercy is seen, which, though it does not totally remit, yet somewhat alleviates, in punishing short of what is deserved. ”

“ Although creation presupposes nothing in the universe; yet it does presuppose something in the knowledge of God. In this way too the idea of justice is preserved in creation; by the production of beings in a manner that accords with the divine wisdom and goodness. And the idea of mercy, also, is preserved in the change of creatures from non-existence to existence. ”

“ Although justice regards act, this does not prevent its being the essence of God; since even that which is of the essence of a thing may be the principle of action. But good does not always regard act; since a thing is called good not merely with respect to act, but also as regards perfection in its essence. ”

“ It is due to God that there should be fulfilled in creatures what His will and wisdom require, and what manifests His goodness. In this respect, God's justice regards what befits Him; inasmuch as He renders to Himself what is due to Himself. It is also due to a created thing that it should possess what is ordered to it; thus it is due to man to have hands, and that other animals should serve him. Thus also God exercises justice, when He gives to each thing what is due to it by its nature and condition. ”

“ We must needs see that God is truly just, in seeing how He gives to all existing things what is proper to the condition of each; and preserves the nature of each in the order and with the powers that properly belong to it. ”

“ Further, the Apostle attributes the conversion of the Jews to justice and truth, but that of the Gentiles to mercy (Rom. 15) . Therefore not in every work of God are justice and mercy.
Objection 3: Further, many just persons are afflicted in this world; which is unjust. Therefore not in every work of God are justice and mercy.
Objection 4: Further, it is the part of justice to pay what is due, but of mercy to relieve misery. Thus both justice and mercy presuppose something in their works: whereas creation presupposes nothing. Therefore in creation neither mercy nor justice is found. ”

“ To each one is due what is his own. Now that which is directed to a man is said to be his own. Thus the master owns the servant, and not conversely, for that is free which is its own cause. In the word debt, therefore, is implied a certain exigence or necessity of the thing to which it is directed. Now a twofold order has to be considered in things: the one, whereby one created thing is directed to another, as the parts of the whole, accident to substance, and all things whatsoever to their end ”