Man’s ultimate happiness consists in the contemplation of truth, for this operation is specific to man and is shared with no other animals. Also it is not directed to any other end since the contemplation of truth is sought for its own sake. In addition, in this operation man is united to higher beings (substances) since this is the only human operation that is carried out both by God and by the separate substances (angels). ( Summa Contra Gentiles, book 3, chapter 37)
It is impossible for any created good to constitute man’s happiness. For happiness is that perfect good which entirely satisfies one’s desire; otherwise it would not be the ultimate end, if something yet remained to be desired. Now the object of the will, i.e., of man’s desire, is what is universally good; just as the object of the intellect is what is universally true. Hence it is evident that nothing can satisfy man’s will, except what is universally good. This is to be found, not in any creature, but in God alone, because every creature has only participated goodness. Therefore, God alone can satisfy the will of man, according to the words of the Psalms (102:5): “Who alone satisfies your desire with good things.” Therefore, God alone constitutes man’s happiness.” (Summa Theologica Part 2. Q.1. Article 8)
Because in man there is first of all an inclination to good in accordance with the nature which he has in common with all substances: inasmuch as every substance seeks the preservation of its own being, according to its nature: and by reason of this inclination, whatever is a means of preserving human life, and of warding off its obstacles, belongs to the natural law.
Secondly, there is in man an inclination to things that pertain to him more specially, according to that nature which he has in common with other animals: and in virtue of this inclination, those things are said to belong to the natural law, “which nature has taught to all animals” [Pandect. Just. I, tit. i], such as sexual intercourse, education of offspring and so forth.
Thirdly, there is in man an inclination to good, according to the nature of his reason, which nature is proper to him: thus man has a natural inclination to know the truth about God, and to live in society: and in this respect, whatever pertains to this inclination belongs to the natural law; for instance, to shun ignorance, to avoid offending those among whom one has to live, and other such things regarding the above inclination.