(_Summa Theologica_, II-II, qq. 79-100.)
2139. The Quasi-Integral Parts of Justice.—The integral parts of a virtue are certain functions necessary for the perfect use of the virtue; for example, memory, perception, docility and quickness are needed for the fullest exercise of prudence (see 1648 sqq.). These parts are called here “quasi-integral,” so as not to be confused with the properly integral parts, or divisions of quantity, in a material composite. In its first use “integral part” is spoken of bodily things; in its derived use of incorporeal things (such as virtues). The two previous articles treated the subjective parts of justice; the present article will begin with a consideration of the integral parts and the opposite sins.
2140. The integral parts of justice are expressed in the words of Psalm xxxiii. 15—“Turn away from evil, and do good”—for the perfectly just man will both establish the equality of justice by giving others their dues, and will preserve that equality by refraining from injuries.
(a) Thus, these integral parts are acts of virtue, and hence the avoidance of evil here is not a purely negative attitude; it includes a positive repudiation by the will of all wish to harm others, and it is exercised especially when one is attacked and yet refuses to resort to injustice.
(b) These integral parts of justice are also distinct, one from the other. The other moral virtues regulate the passions by bringing them to the moderation that lies midway between two evil extremes, and hence in respect to those virtues to turn away from evil is the same thing as to do good. But justice regulates human operations and external things both by reducing them to due equality and also by avoiding that which upsets the equality, and thus in the matter of justice it is one thing to do good, another thing to avoid evil.
(c) These integral parts of justice are also special, that is, they are distinct from other virtues. For, while every virtue turns away from evil and does good, the two acts we are now considering do these things with the express purpose of fulfilling justice. Thus, he who observes the commands and prohibitions of the law in order to render to God and the common good their dues, is perfect in general or legal justice; he who gives to other individuals what is owed them and also avoids doing them injury, is perfect in particular justice. To the two integral parts of justice are opposed the two sins of transgression and omission (see 35-39).
2141. The Potential Parts of Justice.—The potential parts of justice, that is, its annexed virtues, are those good habits that are subsidiary to justice, partaking in some degree, but not entirely, of its nature or activity. We saw above (1664 sqq.) that wise deliberation and wise decision belong to prudence, inasmuch as they are concerned with the government of conduct by reason, but that they fall short of its principal act, which is wise direction, and hence they are counted as potential parts. In like manner, there are a number of virtues which must be assigned to justice, since they regulate man’s will towards others, but which must be considered as its potential parts, because they do not share in one or the other of the two remaining essential notes of strict justice, namely, that a return is given which is equal to a debt, and that the debt is owed on account of a strict or legal right (see 1692).
2142. In the following enumeration are given the chief potential parts of justice in which there is a strict debt, but not an equal repayment.
(a) Thus, to God man owes whatever honor and veneration he manifests, but with all his efforts man can never pay to God a worship that is equal to the debt. Thus, man cannot sufficiently thank God for His benefits: “What return can I make to the Lord for all that He has done for me?” (Psalm cxv. 12). The virtue of religion, therefore, is a potential part of justice.
(b) To parents children cannot make a full return for the benefits of life and upbringing, and the same may be said of one’s country: “A due return is out of the question in honors paid to the gods and to parents ... but a person is considered to be virtuous if he pays such regard as lies within his power” (Aristotle, _Ethics_, lib. VIII, cap. 16). Hence, the virtue of piety is also a potential part of justice. In exceptional cases, however, a child may make an equal or even a greater return to his parents for the benefits received from them; for example, by saving his father from death he makes an equal return for the benefit of life, and by converting his parents to the faith he gives them spiritual life, which is more valuable than the natural life he received from them.