Man is able to recognize good and evil thanks to that discernment of good from evil which he himself carries out by his reason . . .
A freedom which claims to be absolute ends up treating the human body as a raw datum, devoid of any meaning and moral values until freedom has shaped it in accordance with its design. Consequently, human nature and the body appear as presuppositions or preambles,materially necessary for freedom to make its choice, yet extrinsic to the person, the subject and the human act. Their functions would not be able to constitute reference points for moral decisions, because the finalities of these inclinations would be merely "physical"goods, called by some "pre-moral". To refer to them, in order to find in them rational indications with regard to the order of morality, would be to expose oneself to the accusation of physicalism or biologism. In this way of thinking, the tension between freedom and a nature conceived of in a reductive way is resolved by a division within man himself. . .
They also remind us that reason and free will are linked with all the bodily and sense faculties. The person, including the body, is completely entrusted to himself, and it is in the unity of body and soul that the person is the subject of his own moral acts. The person, by the light of reason and the support of virtue, discovers in the body the anticipatory signs, the expression and the promise of the gift of self, in conformity with the wise plan of the Creator. It is in the light of the dignity of the human person — a dignity which must be affirmed for its own sake — that reason grasps the specific moral value of certain goods towards which the person is naturally inclined. And since the human person cannot be reduced to a freedom which is self-designing, but entails a particular spiritual and bodily structure, the primordial moral requirement of loving and respecting the person as an end and never as a mere means also implies, by its very nature, respect for certain fundamental goods, without which one would fall into relativism and arbitrariness. . .
The natural law thus understood does not allow for any division between freedom and nature. Indeed, these two realities are harmoniously bound together, and each is intimately linked to the other. . .
Precisely because of this "truth" the natural law involves universality. Inasmuch as it is inscribed in the rational nature of the person, it makes itself felt to all beings endowed with reason and living in history. In order to perfect himself in his specific order, the person must do good and avoid evil, be concerned for the transmission and preservation of life, refine and develop the riches of the material world, cultivate social life, seek truth, practice good and contemplate beauty. . .