Understand, any 'human act.' No man is happy by the beating of his heart. The proof that happiness consists in an activity of the best in man may be put scholastically thus. -- Being is good. Every being, according to its kind and capacity, asserts itself and aims at maintaining itself: this we may call the self-preservative nisus. Every being, that is capable of development, aims, not at mere maintenance, but at development of self. This effort after development is the Aristotelian phusis. In a conscious and intelligent being, the successful maintenance and development of self is happiness, which might be defined conscia plenitudo essendi. Being (esse) carries power (posse), and power carries act (agere). Power is called by Aristotle the first actuality, and act the second actuality. Being is in its full development when it reaches the second actuality. Man therefore is in the fullness of being, and therefore man is happy, when he is in the best second actuality of which his nature is capable; and that, as Aristotle proves, and St Thomas after him (Chapp. XXVI, XXXVII), is the act of contemplation. -- Whether this demonstration is sufficiently observant of the essential sociableness of human nature, is a point to consider. Is self complete in the individual, and not rather in society? Heaven is the New Jerusalem (Apoc. xxi, 2) and Jerusalem is the city of the great King (Matt.v, 35).

Of God and His Creatures: 3.48