Appendix 1. (26).
Catholic tradition teaches that the natural law is impressed in human beings by nature. According to St. Augustine the eternal law 'is transferred into the human heart not by passing from one place to another but by imprinting itself on the heart, as the image of a ring passes into the wax without abandoning the ring' (De Trinit., bk. 14, c. 15). We note that this passage identifies the natural law with the eternal law, the former being simply a participation in the latter. This can impart much light to students of thought who wish to think as Christians. It shows how, according to the holy Fathers and Church teaching, the fundamental norm of human actions is present within us by the communication of the eternal law. In no way does it come to us from sensations. I say 'according to the holy Fathers' rather than 'according to St. Augustine' because the teaching is common to them all.
St. Thomas himself clearly says that the natural law is not different from the eternal law, but only a participation in that law: 'The natural law is simply the rational creature's participation in the eternal law' (S.T., I-II, q. 91, art. 2). In reply to an objection he says: 'The argument would be acceptable if the natural law, which is only a participation in the eternal law, differed from the eternal law' (ibid., ad primum). Nor must we think that St. Thomas understands the natural law to be some instinct, as some of his passages might indicate. He expressly says that human beings, as opposed to animals, share 'intellectually and rationally' in the eternal mind: this explains why 'participation in the eternal law on the part of the rational creature is properly called law, because law can belong only to reason. Thus in irrational creatures there is no law, except by similitude' (S.T., I-II, q. 91, art. 2, ad 3).
Moreover, we must note how St. Thomas expressly supposes that, although children cannot make use of the natural law, it is nevertheless impressed as a habit in them, as the principle of knowledge is: 'Sometimes we cannot use what we possess habitually because of some impediment, just as we cannot use the habit of knowledge during sleep. In the same way, the very young child cannot make use of the habit of intelligence, of principles, or of natural law which is habitually present within it' (S.T., I-II, q. 94, art. 1, ad 3). Clearly, then, St. Thomas maintains that the child possesses the moral law in the way that an educated man who is asleep, or not actually thinking, possesses knowledge. It is not possible, therefore, to hold that St. Thomas is a sensist.
To show that I am in complete agreement with St. Thomas and St. Augustine I will add two more observations.
First, I have supposed that the light of reason, inserted in us by nature, is the supreme moral rule, which is precisely what St. Thomas teaches. After referring to the passage of the Psalmist, 'The light of your face, O Lord, is signed upon us' (Ps 4, [7 (Douai)]), he adds: 'as if the LIGHT OF natural REASON, by which we discern good from evil, and which belongs to the natural law, is simply the impression of divine light in us. Hence it is clear that the natural law is simply the rational creature's participation in the eternal law' (S.T., I-II, q. 91, art. 2).
Secondly, I call this light of reason truth, which truth, as I showed, is being as intuited. This is precisely what St. Augustine teaches. After asking how the wicked see the moral rules which even they use to reprove evil actions, he replies: 'In the book of that light called TRUTH which describes every just law' (De Trin., bk. 14, c. 15). Cf. also PE, 8-12.