It is natural ( instinctive) for both animals and men to form communities for mutual help. But man is also intelligent: he seeks to understand the world be lives in, and himself. With this power geared to being as such, he turns toward totality of being while perceiving the fragmentation of beings, toward infinity of being while perceiving the limitation of beings, toward perfection f being while perceiving the perfectibility of beings, toward essential being while perceiving the mere participants in, or receivers of being.
This thrust of man’s intelligence if what leads him to both the acceptance of God’s existence as the only Being who really is in the fullness of being (neither static nor dynamic , but eternal), and the acceptance of an eternal law(reflection of the infinite wisdom of the Creator), i.e., a universal and ever-valid order (the moral order for rational beings: St. Thomas Aquinas defines the natural moral law as the participation of the eternal law in the rational creature) binding all beings to God as their last end, but in the case of man in a conscious and self-determining way: man is the only being in the visible universe who has the power to know and to love God, and to obey him freely.
Thus, man seeks a perfect social order based on reason and love (justice and benevolence, rights and duties and friendship), because with his rationality, i.e., with his capacity to understand what is ultimately good and be motivated by it with the self-determination of his free will, he transcends society as such and is prior to it (society is for man, not man for society). Consequently he tries to embody in his social life as faithfully a possible that ordering to the last end which he can perceive with his reason.