Now, as it is in the Summary for theological purposes that the whole practical philosophy of Aquinas is contained, it is to be inferred that he regarded the subject of Ethics as not on the same level with other departments of philosophy.

Moreover, even when he is not appealing to Scripture, he is seen to display what is for him a most unusual tendency to desert Aristotle, at the really critical moments, for Plato or Plotinus, or any other authority of a more theological cast.
In the (unfinished) _Summa Theologiae_, the Ethical views and cognate questions occupy the two sections of the second part--the so-called _prima_ and _secunda secundae_.

He begins, in the Aristotelian fashion, by seeking an ultimate end of human action, and finds it in the attainment of the highest good or happiness.

But as no created thing can answer to the idea of the highest good, it must be placed in God.
God, however, as the highest good, can only be the object, in the search after human happiness, for happiness in itself is a state of the mind or act of the soul.

The question then arises, "what sort of act ?" Does it fall under the Will or under the Intelligence?
The answer is, Not under the will, because happiness is neither desire nor pleasure, but _consecutio_, that is, a possessing.