Reply Obj. 3: This argument proves that the natural law is held habitually; and this is granted.
To the argument advanced in the contrary sense we reply that sometimes a man is unable to make use of that which is in him habitually, on account of some impediment: thus, on account of sleep, a man is unable to use the habit of science. In like manner, through the deficiency of his age, a child cannot use the habit of understanding of principles, or the natural law, which is in him habitually. ________________________
SECOND ARTICLE [I-II, Q. 94, Art. 2]
Whether the Natural Law Contains Several Precepts, or Only One?
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 (Q. 92, A. 2). If therefore there were many precepts of the natural law, it would follow that there are also many natural laws.
Obj. 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.
Obj. 3: Further, law is something pertaining to reason, as stated above (Q. 90, A. 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 (Q. 91, A. 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.