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"It is through conscience that human beings see and recognize the demands of the divine law. They are bound to follow their conscience faithfully in everything they do."
Religious Liberty, no 2.

Excerpts from the Summa Theologica

Is there in us a natural law?

Thomas Aquinas, Summa Theologica I-II, question 91, article 2.

Objection 1. It would seem that there is no  natural law in us. Because man is governed sufficiently by the  eternal  law: for  Augustine says (De Lib. Arb. i) that "the eternal  law is that by which it is right that all things should be most orderly." But  nature does not abound in superfluities as neither does she fail in necessaries. Therefore no  law is  natural to  man.

Objection 2. Further, by the  law  man is directed, in his acts, to the end, as stated above (Question 90, Article 2). But the directing of  human acts to their end is not a function of  nature, as is the case in irrational creatures, which act for an end solely by their  natural  appetite; whereas  man acts for an end by his  reason and will. Therefore no  law is  natural to  man.

Objection 3. Further, the more a man is free, the less is he under the  law. But  man is freer than all the animals, on account of his  free-will, with which he is endowed above all other animals. Since therefore other animals are not subject to a  natural law, neither is  man subject to a  natural law.

On the contrary, A  gloss on Romans 2:14: "When the Gentiles, who have not the  law, do by  naturethose things that are of the  law," comments as follows: "Although they have no written  law, yet they have the  natural law, whereby each one  knows, and is conscious of, what is  good and what is  evil."

I answer that, As stated above (90, 1, ad 1),  law, being a rule and measure, can be in a  person in two ways: in one way, as in him that rules and measures; in another way, as in that which is ruled and measured, since a thing is ruled and measured, in so far as it partakes of the rule or measure. Wherefore, since all things subject to  Divine providence are ruled and measured by the  eternal  law, as was stated above (Article 1); it is evident that all things partake somewhat of the  eternal  law, in so far as, namely, from its being imprinted on them, they derive their respective inclinations to their properacts and ends.

Now among all others, the rational creature is subject to  Divine providence in the most excellent way, in so far as it partakes of a share of  providence, by being provident both for itself and for others. Wherefore it has a share of the  Eternal  Reason, whereby it has a  natural inclination to its proper act and end: and this participation of the  eternal  law in the rational creature is called the  natural law.

Hence the Psalmist after saying (Psalm 4:6): "Offer up the sacrifice of  justice," as though someone asked what the works of  justice are, adds: "Many say, Who showeth us  good things?"

In answer to which question he says: "The light of Thy countenance, O Lord, is signed upon us": thus implying that the light of  natural  reason, whereby we discern what is  good and what is  evil, which is the function of the  natural law, is nothing else than an imprint on us of the Divine light.

It is therefore evident that the  natural law is nothing else than the rational creature's participation of the  eternal  law.

Reply to Objection 1. This argument would hold, if the  natural law were something different from the eternal  law: whereas it is nothing but a participation thereof, as stated above.

Reply to Objection 2. Every  act of  reason and will in us is based on that which is according to  nature, as stated above (Question 10, Article 1): for every act of reasoning is based on principles that are known  naturally, and every act of  appetite in respect of the means is derived from the  natural  appetitein respect of the last end. Accordingly the first direction of our acts to their end must needs be in virtue of the  natural law.

Reply to Objection 3. Even irrational animals partake in their own way of the  Eternal  Reason, just as the rational creature does. But because the rational creature partakes thereof in an  intellectual and rational manner, therefore the participation of the  eternal  law in the rational creature is properly called a  law, since a  law is something pertaining to reason, as stated above (Question 90, Article 1). Irrational creatures, however, do not partake thereof in a rational manner, wherefore there is no participation of the  eternal  law in them, except by way of similitude.

Thomas Aquinas, Summa Theologica I-II, question 91, article 2.

Has natural law been promulgated?

The natural law is promulgated by the very fact that God instilled it into man's mind so as to be known by him naturally.

Thomas Aquinas, Summa Theologica I-II, question 90, article 4, ad 1.

Is natural law the same in all people ?

Thomas Aquinas, Summa Theologica I-II, question 94, article 4.

Objection 1. It would seem that the  natural law is not the same in all. For it is stated in the Decretals(Dist. i) that "the  natural law is that which is contained in the  Law and the Gospel." But this is not common to all  people ; because, as it is written (Romans 10:16), "all do not obey the gospel." Therefore the  natural law is not the same in all  people .

Objection 2. Further, "Things which are according to the  law are said to be  just," as stated in Ethic. v. But it is stated in the same book that nothing is so universally just as not to be subject to change in regard to some people . Therefore even the  natural law is not the same in all  people .

Objection 3. Further, as stated above (2,3), to the  natural law belongs everything to which a man is inclined according to his  nature. Now different people are  naturally inclined to different things; some to the desire of pleasures, others to the desire of honors, and other people to other things. Therefore there is not one  natural law for all.

On the contrary,  Isidore says (Etym. v, 4): "The  natural law is common to all  nations."

I answer that, As stated above (2,3), to the  natural law belongs those things to which a man is inclined  naturally: and among these it is proper to  man to be inclined to act according to  reason. Now the process of reason is from the common to the specific, as stated in Phys. i. The speculative reason, however, is differently situated in this matter, from the practical reason. For, since the speculative reason is busied chiefly with the  necessary things, which cannot be otherwise than they are, its proper conclusions, like the universal principles, contain the  truth without fail.

The practical reason, on the other hand, is busied with contingent [= specific and transient] matters, about which  human actions are concerned: and consequently, although there is  necessity in the general principles, the more we descend to matters of detail, the more frequently we encounter deviations.

Accordingly then in speculative matters  truth is the same in all  people , both as to principles and as to conclusions: although the  truth is not  known to all as regards the conclusions, but only as regards the principles which are called common notions.

But in matters of action,  truth or practical rectitude is not the same for all, as to matters of detail, but only as to the general principles: and where there is the same rectitude in matters of detail, it is not equally known to all.

It is therefore evident that, as regards the general principles whether of speculative or of practical reason,  truth or rectitude is the same for all, and is equally  known by all. As to the proper conclusions of the speculative reason, the  truth is the same for all, but is not equally  known to all: thus it is  true for all that the three angles of a triangle are together equal to two right angles, although it is not known to all.

But as to the proper conclusions of the practical reason, neither is the  truth or rectitude the same for all, nor, where it is the same, is it equally  known by all. Thus it is right and  true for all to act according to reason: and from this principle it follows as a proper conclusion, that goods entrusted to another should be restored to their owner. Now this is  true for the majority of cases: but it may happen in a particular case that it would be injurious, and therefore unreasonable, to restore goods held in trust; for instance, if they are claimed for the purpose of fighting against one's country. And this principle will be found todeviate the more, according as we descend further into detail, e.g. if one were to say that goods held in trust should be restored with such and such a guarantee, or in such and such a way; because the greater the number of  conditions added, the greater the number of ways in which the principle may deviate, so that it be not right to restore or not to restore.

Consequently we must say that the  natural law, as to general principles, is the same for all, both as to rectitude and as to  knowledge. But as to certain matters of detail, which are conclusions, as it were, of those general principles, it is the same for all in the majority of cases, both as to rectitude and as to knowledge; and yet in some few cases it may deviate, both as to rectitude, by reason of certain obstacles (just as natures subject to generation and corruption fail in some few cases on account of some obstacle), and as to  knowledge, since in some the reason is perverted by passion, or  evil  habit, or an evil disposition of  nature; thus formerly,  theft, although it is expressly contrary to the  natural law, was not considered wrong among the Germans, as Julius Caesar relates (De Bello Gall. vi).

Reply to Objection 1. The meaning of the sentence quoted is not that whatever is contained in the Law and the Gospel belongs to the  natural law, since they contain many things that are above  nature; but that whatever belongs to the  natural law is fully contained in them. Wherefore Gratian, after saying that "the  natural law is what is contained in the  Law and the Gospel," adds at once, by way of example, "by which everyone is commanded to do to others as he would be done by."

Reply to Objection 2. The saying of the  Philosopher is to be understood of things that are  naturally just, not as general principles, but as conclusions drawn from them, having rectitude in the majority of cases, but failing in a few.

Reply to Objection 3. As, in  man, reason rules and commands the other powers, so all the  naturalinclinations belonging to the other powers must needs be directed according to  reason. Wherefore it is universally right for all  people , that all their inclinations should be directed according to  reason.

Thomas Aquinas, Summa Theologica I-II, question 94, article 4.

Can the natural law can be changed?

Thomas Aquinas, Summa Theologica I-II, question 94, article 5.

Objection 1. It would seem that the  natural law can be changed. Because on Sirach 17:9, "He gave them instructions, and the  law of life," the  gloss says: "He wished the  law of the letter to be written, in order to correct the  law of  nature." But that which is corrected is changed. Therefore the  natural law can be changed.

Objection 2. Further, the slaying of the innocent,  adultery, and  theft are against the  natural law. But we find these things changed by  God: as when  God commanded  Abraham to slay his innocent son (Genesis 22:2); and when he ordered the  Jews to borrow and purloin the vessels of the Egyptians(Exodus 12:35); and when He commanded Osee to take to himself "a wife of fornications" (Hosea 1:2). Therefore the  natural law can be changed.

Objection 3. Further,  Isidore says (Etym. 5:4) that "the possession of all things in common, and universal freedom, are matters of  natural law." But these things are seen to be changed by  human laws. Therefore it seems that the  natural law is subject to change.

On the contrary, It is said in the Decretals (Dist. v): "The  natural law dates from the creation of the rational creature. It does not vary according to time, but remains unchangeable."

I answer that, A change in the  natural law may be understood in two ways. First, by way of addition. In this sense nothing hinders the  natural law from being changed: since many things for the benefit of human life have been added over and above the  natural law, both by the Divine law and by  human laws.

Secondly, a change in the  natural law may be understood by way of subtraction, so that what previously was according to the  natural law, ceases to be so. In this sense, the  natural law is altogether unchangeable in its first principles: but in its secondary principles, which, as we have said (4), are certain detailed proximate conclusions drawn from the first principles, the  natural law is not changed so that what it prescribes be not right in most cases. But it may be changed in some particular cases of rare occurrence, through some special  causes hindering the observance of such precepts, as stated above (Article 4).

Reply to Objection 1. The written  law is said to be given for the correction of the  natural law, either because it supplies what was wanting to the  natural law; or because the  natural law was perverted in the hearts of some people , as to certain matters, so that they esteemed those things  good which are naturally  evil; which perversion stood in need of correction.

Reply to Objection 2. All people alike, both guilty and innocent, die the death of  nature: which death of nature is inflicted by the power of  God on account of  original sin, according to 1 Samuel 2:6: "The Lord killeth and maketh alive." Consequently, by the command of  God, death can be inflicted on any  man, guilty or innocent, without any  injustice whatever. In like manner  adultery is intercourse with another's wife; who is allotted to him by the  law emanating from  God. Consequently intercourse with any woman, by the command of  God, is neither  adultery nor fornication. The same applies to  theft, which is the taking of another's property. For whatever is taken by the command of  God, to Whom all things belong, is not taken against the will of its owner, whereas it is in this that  theft consists. Nor is it only in human things, that whatever is commanded by  God is right; but also in  natural things, whatever is done by  God, is, in some way,  natural, as stated in the I, 105, 6, ad 1.

Reply to Objection 3. A thing is said to belong to the  natural law in two ways. First, because  nature inclines thereto: e.g. that one should not do harm to another. Secondly, because  nature did not bring in the contrary: thus we might say that for  man to be naked is of the  natural law, because  nature did not give him clothes, but art invented them. In this sense, "the possession of all things in common and universal freedom" are said to be of the  natural law, because, to wit, the distinction of possessions and slavery were not brought in by  nature, but devised by  human  reason for the benefit of  human life. Accordingly the  law of  nature was not changed in this respect, except by addition.

Thomas Aquinas, Summa Theologica I-II, question 94, article 5.

Thomas Aquinas

People should know the truth!

St Thomas Aquinas (1225-1274) is considered the outstanding medieval authority on Natural Law.

Unfortunately, later Christian writers overlooked his valuable insights. They reverted to a more wooden, physical interpretation.

Of course, Thomas himself came to some wrong conclusions on sexual morality on account of his ignorance of biological facts. See Summa Contra Gentiles III 122.