This paper examines the differing perspectives of Aristotle and Aquinas on law and justice. It contrasts Aristotle's view that justice is an inherent characteristic of human character rooted in reason and natural order with Aquinas's theologically grounded understanding of law as an ordinance of reason directed toward the common good and governed by divine providence. The paper surveys Aristotle's two categories of law — natural and conventional — alongside Aquinas's four-part typology of eternal, natural, human, and divine law. It also addresses distributive justice, corrective justice, and the principle of reciprocity as articulated by Aristotle, concluding with a comparison of how each thinker understood mankind's moral capacity and the necessity of law.
Aristotle and Aquinas diverged significantly on law and justice. Aristotle held that justice was inherent to the individual — a matter of reasoning or an inner sense of right and wrong. He believed that law should be grounded in a natural, divine order of some kind, and that this cosmic order is what vests law with binding authority.
Aristotle also believed, as did Plato, that law's core function is to compensate for the erratic judgment of men, which varies from one person or culture to another. In one example drawing upon Plato's Socrates, Aristotle noted the passions of people and their randomness, which can nonetheless be brought together by reason and focused toward a higher purpose. Aristotle's view of proper political systems included kingship, aristocracy, and polity — all of which are capable of overseeing the common interest. He held that tyranny is opposed to kingship, oligarchy is opposed to aristocracy, and democracy is opposed to polity.
Aquinas, however, objected that law is "not something pertaining to reason," basing this on Romans 7:23, in which the apostle states that he sees another law in his members. As Aquinas argued: "I see another law in my members… But nothing pertaining to reason is in the members; since the reason does not make use of a bodily organ. Therefore, law is not something pertaining to reason" (Aquinas, p. 1).
Aquinas further argued that all that remains beyond reason is "power, habit and act" (p. 1), yet law is not a habit of reason — since the habits of reason are the intellectual virtues — nor is it an act of reason, since on that account the law would cease to exist whenever an individual is asleep. Aquinas went on to state that the law causes those subject to it to act rightly, but that this is causative of the will to move to act, and therefore pertains not to individual reason but to individual will. Law has the power to command and the power to forbid, but it is reason that does the commanding — so in that sense the law does belong to reason.
According to Aquinas, law "is a rule and measure of acts" that can either motivate a man to act or restrain him from acting. Aristotle, by contrast, held that all decent individuals share certain agreements — including that the dead should be treated with respect and that laws should be obeyed.
Aristotle also grappled with the practical problem of defining justice. He sought to avoid both idealist and relativist definitions, aiming instead for an empirical account. Justice, for Aristotle, is a characteristic of a person's character grounded in degrees of accountability — distinguishing voluntary from involuntary actions, and excusable ignorance from willful ignorance. He held that amorality is the inability to discern right from wrong, and that justice allows for equity; injustice arises when rules are applied in their strictest sense without regard for circumstance.
According to Aristotle, there are two types of law: (1) natural law, and (2) conventional law. Natural law is applicable everywhere in the world, while conventional law is established by people through legislation. Aristotle held that the unequal and the unlawful are not identical — everything that is unequal is truly unlawful, but not everything that is unlawful is unequal.
Justice in a court of law involves the judge attempting to establish equity by equalizing the situation. For example, an individual who has attacked another is given a sentence intended to produce an equitable resolution. Justice can be said to have been achieved when legal rules and practices are "economically efficient" (Parisi and Rowley, n.d.).
In Aquinas's framework, there are correspondingly multiple levels of law. Regarding natural law, Aquinas raised several objections: first, that there is no natural law in the human being since man is governed by eternal law; second, that since law directs man in his actions, law is not a function of nature; and third, that if man is free, he is not subject to natural law in the way animals are subject to natural instinct. Aquinas nevertheless concluded that all things are subject to divine law and measured by eternal law, and that man has a share of "eternal reason" — instilling in him a natural inclination to act properly and with proper intentions.
"Aquinas's objections and responses on law's purpose"
"Aristotle's three forms of justice explained"
Aquinas holds the belief, as written in the gospels, that man is inherently evil and that law is required to motivate man toward good actions. He holds that man is incapable of fairly judging other men and that the law is needed to motivate equity and justice in society. Aristotle, on the other hand, believed that man inherently knew the equitable and just way he should act — whether or not he chose to act in that manner. Aquinas counters this view by noting that if Aristotle were correct, the Psalmist David would not have prayed to God to instill his righteousness and his law within him.
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