Thomas Aquinas, answered from the texts and cited to the page.
The charge of smuggling deserves a precise answer, because it is not one objection but three, and they do not all land in the same place. The first precept — good is to be pursued, evil avoided — is not derived from anything at all, and so cannot smuggle in what it has not inferred. It is self-evident in the practical order as the principle of non-contradiction is in the speculative: every judgment of practical reason proceeds from principles known naturally, just as every judgment of speculative reason proceeds from the natural knowledge of first principles.1
The ought is not manufactured at the end of an inference from observed human tendencies; it is present the moment the intellect grasps anything as a good to be pursued. Hume's objection, that no is entails an ought, is aimed at a move I do not make. The further precepts are a different matter, and here the distinction between two modes of derivation is everything.2
Some things follow from the general principles as conclusions follow from premises — "one must not kill" follows from "do harm to no man" as a demonstrated conclusion, and has force from the natural law itself, not merely from the human law that enacts it.3 Other determinations are not conclusions but specifications: the natural law requires that evil-doers be punished, but whether by fine or imprisonment or exile is left open, and the human legislator fills that space by way of determination.4
Laws derived in the second mode have no force except from the human law that posits them; laws derived in the first mode carry their natural-law force with them even when enacted positively. Now the universality claim. The general principles are the same for all and known to all. But the general principles cannot be applied to all men in the same way, on account of the great variety of human affairs, and hence arises the diversity of positive laws among various peoples.5
A theory that predicted moral unanimity at the level of determinate enactments would be refuted by an hour's reading of the jurists. What I claim is a common root and divergent branches, not a single trunk that reaches into every detail of every legal system. There is a further gradation within the moral precepts themselves that the objector should attend to.
Some matters connected with human action are so evident that after very little consideration one can approve or disapprove of them by means of the first principles. Others cannot be judged without much consideration of circumstances, and only the wise are competent to do this carefully — just as not everyone can follow the particular conclusions of the sciences, but only those versed in philosophy.
And there are matters of which man cannot judge at all unless helped by divine instruction.6 The universality of natural law does not require that every man reach every moral conclusion on his own; it requires that the principles from which all such conclusions must be drawn are available to reason as reason. What is genuinely under pressure is the move from the general precepts to the very determinate ones — and I concede that move is the work of human law, admits of variation, and requires the judgment of the prudent rather than a mechanical deduction.
The Philosopher says that in such matters we ought to pay as much attention to the undemonstrated sayings of persons who surpass us in experience, age, and prudence as to their demonstrations.7 That is not a retreat from reason; it is what reason looks like when it operates in the domain of contingent particulars rather than necessary truths.
The craftsman who determines the general form of a house to some particular shape is not abandoning the principles of architecture; he is applying them where they do not apply themselves.8 The force of law, at every level, depends on the extent of its justice; and what deflects from the law of nature is no longer law but a perversion of law.9
That is the boundary. Within it, there is room — and need — for human determination, and the diversity that follows from determination is not evidence against the universality of the root.
Natural law is the rational creature's participation in the eternal law.10 Its first precept — good is to be pursued, evil avoided — is self-evident, and the further precepts follow the order of the natural inclinations: to preserve being, to the union of the sexes and the rearing of offspring, to know the truth about God and to live in society.1112
every judgment of practical reason proceeds from principles known naturally, as stated above (Q. 94, AA. 2, 4): from which principles one may proceed in various ways to judge of various matters.Summa Theologica Part I-II, Prima Secundae
it must be noted that something may be derived from the natural law in two ways: first, as a conclusion from premises, secondly, by way of determination of certain generalities.Summa Theologica Part I-II, Prima Secundae
those things which are derived in the first way, are contained in human law not as emanating therefrom exclusively, but have some force from the natural law also.Summa Theologica Part I-II, Prima Secundae
those things which are derived in the second way, have no other force than that of human law.Summa Theologica Part I-II, Prima Secundae
The general principles of the natural law cannot be applied to all men in the same way on account of the great variety of human affairs: and hence arises the diversity of positive laws among various people.Summa Theologica Part I-II, Prima Secundae
some matters connected with human actions are so evident, that after very little consideration one is able at once to approve or disapprove of them by means of these general first principles: while some matters cannot be the subject of judgment without much consideration of the various circumstances, which all are not competent to do carefully, but only those who are wise.Summa Theologica Part I-II, Prima Secundae
we ought to pay as much attention to the undemonstrated sayings and opinions of persons who surpass us in experience, age and prudence, as to their demonstrations.Summa Theologica Part I-II, Prima Secundae
the craftsman needs to determine the general form of a house to some particular shape.Summa Theologica Part I-II, Prima Secundae
if in any point it deflects from the law of nature, it is no longer a law but a perversion of law.Summa Theologica Part I-II, Prima Secundae
Wherefore it has a share of the Eternal Reason, whereby it has a natural inchnation to its proper act and end: and this 2^^i£i£5ii2IL of the eternal law in the rational creature is called the natural law. Hence the Psalmist after saying (Ps. iv. 6): Offer up the sacrifice of justice, as though someone asked what the works of justice are, adds: Many say, Who showeth us good things?Summa Theologica Part I II, Prima Secundae, vol 8, pp. 12–13
Hence tMs is the first precept of law, that good is to be done and ensued, and evil is to be avoided. All other precepts of the natural law are based upon this: so that whatever the practical reason naturally apprehends as man's good (or evil) belongs to the precepts of the natural law as something to be done or avoided. A, Since, however, good has the nature of an end, and evil^ ^ o. 94. Art.Summa Theologica Part I II, Prima Secundae, vol 8, pp. 44–45
But the end of the Divine government is God Himself, and His law is not distinct from Himself. Wherefore the eternal law is not ordained to another end. Second Article, whether there is in us a natural law? We proceed thus to the Second Article: — Ohjection 1. It seems that there is no natural law in us. Because man is governed sufficiently by the eternal law: for Augustine says (De Lib. Arh.Summa Theologica Part I II, Prima Secundae, vol 8, pp. 10–11
The most durable and the most disputed part of the system, and the part most often accused of deriving an ought from an is.