Man acts according to his nature when he subordinates his physical appetites to reason. There is purpose, order and intelligence in the universe, stemming from the first being, the unmoved Mover, God. The faculty of reason in every man can be trained, through the principles of logic, to reason toward true conclusions. Truth is objective. For example, a true proposition does not depend upon the mind of the individual man for its existence.
Truths exist in nature and are discoverable by the reason of man.
The highest good to which man may aspire is happiness. A truly happy life can be assessed only upon its completion. A man lives happily when his actions are in accordance with his nature. Man's spiritual nature is superior to his physical nature. The highest good for any man is the activity of his soul.