Thomas Aquinas, Summa Theologiae

Objection 1: It would seem that the natural law contains, not several precepts, but one only. For law is a kind of precept, as stated above (Question 92, Article 2). If therefore there were many precepts of the natural law, it would follow that there are also many natural laws.

Objection 2: Further, the natural law is consequent to human nature. But human nature, as a whole, is one; though, as to its parts, it is manifold. Therefore, either there is but one precept of the law of nature, on account of the unity of nature as a whole; or there are many, by reason of the number of parts of human nature. The result would be that even things relating to the inclination of the concupiscible faculty belong to the natural law.

Objection 3: Further, law is something pertaining to reason, as stated above (Question 90, Article 1). Now reason is but one in man. Therefore there is only one precept of the natural law.

On the contrary, The precepts of the natural law in man stand in relation to practical matters, as the first principles to matters of demonstration. But there are several first indemonstrable principles. Therefore there are also several precepts of the natural law.

I answer that, As stated above (Question 91, Article 3), the precepts of the natural law are to the practical reason, what the first principles of demonstrations are to the speculative reason; because both are self-evident principles. Now a thing is said to be self-evident in two ways: first, in itself; secondly, in relation to us. Any proposition is said to be self-evident in itself, if its predicate is contained in the notion of the subject: although, to one who knows not the definition of the subject, it happens that such a proposition is not self-evident. For instance, this proposition, "Man is a rational being," is, in its very nature, self-evident, since who says "man," says "a rational being": and yet to one who knows not what a man is, this proposition is not self-evident. Hence it is that, as Boethius says (De Hebdom.), certain axioms or propositions are universally self-evident to all; and such are those propositions whose terms are known to all, as, "Every whole is greater than its part," and, "Things equal to one and the same are equal to one another." But some propositions are self-evident only to the wise, who understand the meaning of the terms of such propositions: thus to one who understands that an angel is not a body, it is self-evident that an angel is not circumscriptively in a place: but this is not evident to the unlearned, for they cannot grasp it.

Now a certain order is to be found in those things that are apprehended universally. For that which, before aught else, falls under apprehension, is "being," the notion of which is included in all things whatsoever a man apprehends. Wherefore the first indemonstrable principle is that "the same thing cannot be affirmed and denied at the same time," which is based on the notion of "being" and "not-being": and on this principle all others are based, as is stated in Metaph. iv, text. 9. Now as "being" is the first thing that falls under the apprehension simply, so "good" is the first thing that falls under the apprehension of the practical reason, which is directed to action: since every agent acts for an end under the aspect of good. Consequently the first principle of practical reason is one founded on the notion of good, viz. that "good is that which all things seek after." Hence this is the first precept of law, that "good is to be done and pursued, 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.

Since, however, good has the nature of an end, and evil, the nature of a contrary, hence it is that all those things to which man has a natural inclination, are naturally apprehended by reason as being good, and consequently as objects of pursuit, and their contraries as evil, and objects of avoidance. Wherefore according to the order of natural inclinations, is the order of the precepts of the natural law. Because in man there is first of all an inclination to good in accordance with the nature which he has in common with all substances: inasmuch as every substance seeks the preservation of its own being, according to its nature: and by reason of this inclination, whatever is a means of preserving human life, and of warding off its obstacles, belongs to the natural law. Secondly, there is in man an inclination to things that pertain to him more specially, according to that nature which he has in common with other animals: and in virtue of this inclination, those things are said to belong to the natural law, "which nature has taught to all animals" , such as sexual intercourse, education of offspring and so forth. Thirdly, there is in man an inclination to good, according to the nature of his reason, which nature is proper to him: thus man has a natural inclination to know the truth about God, and to live in society: and in this respect, whatever pertains to this inclination belongs to the natural law; for instance, to shun ignorance, to avoid offending those among whom one has to live, and other such things regarding the above inclination.

Reply to Objection 1: All these precepts of the law of nature have the character of one natural law, inasmuch as they flow from one first precept.

Reply to Objection 2: All the inclinations of any parts whatsoever of human nature, e.g. of the concupiscible and irascible parts, in so far as they are ruled by reason, belong to the natural law, and are reduced to one first precept, as stated above: so that the precepts of the natural law are many in themselves, but are based on one common foundation.

Reply to Objection 3: Although reason is one in itself, yet it directs all things regarding man; so that whatever can be ruled by reason, is contained under the law of reason.

Articulus 2

Argumentum 1

Ad secundum sic proceditur. Videtur quod lex naturalis non contineat plura praecepta, sed unum tantum.

Lex enim continetur in genere praecepti, ut supra habitum est.

Si igitur essent multa praecepta legis naturalis, sequeretur quod etiam essent multae leges naturales.

Argumentum 2

Praeterea, lex naturalis consequitur hominis naturam.

Sed humana natura est una secundum totum, licet sit multiplex secundum partes. Aut ergo est unum praeceptum tantum legis naturae, propter unitatem totius, aut sunt multa, secundum multitudinem partium humanae naturae. Et sic oportebit quod etiam ea quae sunt de inclinatione concupiscibilis, pertineant ad legem naturalem.

Argumentum 3

Praeterea, lex est aliquid ad rationem pertinens, ut supra dictum est.

Sed ratio in homine est una tantum.

Ergo solum unum praeceptum est legis naturalis.

Sed Contra

Sed contra est quia sic se habent praecepta legis naturalis in homine quantum ad operabilia, sicut se habent prima principia in demonstrativis. Sed prima principia indemonstrabilia sunt plura. Ergo etiam praecepta legis naturae sunt plura.

Corpus

Respondeo dicendum quod, sicut supra dictum est, praecepta legis naturae hoc modo se habent ad rationem practicam, sicut principia prima demonstrationum se habent ad rationem speculativam, utraque enim sunt quaedam principia per se nota. Dicitur autem aliquid per se notum dupliciter, uno modo, secundum se; alio modo, quoad nos. Secundum se quidem quaelibet propositio dicitur per se nota, cuius praedicatum est de ratione subiecti, contingit tamen quod ignoranti definitionem subiecti, talis propositio non erit per se nota.

Sicut ista propositio, homo est rationale, est per se nota secundum sui naturam, quia qui dicit hominem, dicit rationale, et tamen ignoranti quid sit homo, haec propositio non est per se nota. Et inde est quod, sicut dicit boetius, in libro de hebdomad., quaedam sunt dignitates vel propositiones per se notae communiter omnibus, et huiusmodi sunt illae propositiones quarum termini sunt omnibus noti, ut, omne totum est maius sua parte, et, quae uni et eidem sunt aequalia, sibi invicem sunt aequalia.

Quaedam vero propositiones sunt per se notae solis sapientibus, qui terminos propositionum intelligunt quid significent, sicut intelligenti quod Angelus non est corpus, per se notum est quod non est circumscriptive in loco, quod non est manifestum rudibus, qui hoc non capiunt. In his autem quae in apprehensione omnium cadunt, quidam ordo invenitur.

Nam illud quod primo cadit in apprehensione, est ens, cuius intellectus includitur in omnibus quaecumque quis apprehendit.

Et ideo primum principium indemonstrabile est quod non est simul affirmare et negare, quod fundatur supra rationem entis et non entis, et super hoc principio omnia alia fundantur, ut dicitur in IV metaphys..

Sicut autem ens est primum quod cadit in apprehensione simpliciter, ita bonum est primum quod cadit in apprehensione practicae rationis, quae ordinatur ad opus, omne enim agens agit propter finem, qui habet rationem boni. Et ideo primum principium in ratione practica est quod fundatur supra rationem boni, quae est, bonum est quod omnia appetunt.

Hoc est ergo primum praeceptum legis, quod bonum est faciendum et prosequendum, et malum vitandum. Et super hoc fundantur omnia alia praecepta legis naturae, ut scilicet omnia illa facienda vel vitanda pertineant ad praecepta legis naturae, quae ratio practica naturaliter apprehendit esse bona humana. Quia vero bonum habet rationem finis, malum autem rationem contrarii, inde est quod omnia illa ad quae homo habet naturalem inclinationem, ratio naturaliter apprehendit ut bona, et per consequens ut opere prosequenda, et contraria eorum ut mala et vitanda.

Secundum igitur ordinem inclinationum naturalium, est ordo praeceptorum legis naturae.

Inest enim primo inclinatio homini ad bonum secundum naturam in qua communicat cum omnibus substantiis, prout scilicet quaelibet substantia appetit conservationem sui esse secundum suam naturam. Et secundum hanc inclinationem, pertinent ad legem naturalem ea per quae vita hominis conservatur, et contrarium impeditur. Secundo inest homini inclinatio ad aliqua magis specialia, secundum naturam in qua communicat cum ceteris animalibus. Et secundum hoc, dicuntur ea esse de lege naturali quae natura omnia animalia docuit, ut est coniunctio maris et feminae, et educatio liberorum, et similia. Tertio modo inest homini inclinatio ad bonum secundum naturam rationis, quae est sibi propria, sicut homo habet naturalem inclinationem ad hoc quod veritatem cognoscat de deo, et ad hoc quod in societate vivat. Et secundum hoc, ad legem naturalem pertinent ea quae ad huiusmodi inclinationem spectant, utpote quod homo ignorantiam vitet, quod alios non offendat cum quibus debet conversari, et cetera huiusmodi quae ad hoc spectant.

Ad 1

Ad primum ergo dicendum quod omnia ista praecepta legis naturae, inquantum referuntur ad unum primum praeceptum, habent rationem unius legis naturalis.

Ad 2

Ad secundum dicendum quod omnes inclinationes quarumcumque partium humanae naturae, puta concupiscibilis et irascibilis, secundum quod regulantur ratione, pertinent ad legem naturalem, et reducuntur ad unum primum praeceptum, ut dictum est.

Et secundum hoc, sunt multa praecepta legis naturae in seipsis, quae tamen communicant in una radice.

Ad 3

Ad tertium dicendum quod ratio, etsi in se una sit, tamen est ordinativa omnium quae ad homines spectant. Et secundum hoc, sub lege rationis continentur omnia ea quae ratione regulari possunt.