Summa Theologica

Thus far Aquinas has treated of God and the things proceeding directly from God’s will—all created things, including man. Now he discusses man in and of himself, as a free agent who is the master of his own actions, always with reference to God as man’s proper end. 

Man is different from other creatures in that he possesses reason and will. Reason and will are directed to an end, and thus it is proper to man to act for the sake of an end. (Irrational creatures like plants also act for the sake of an end, not consciously but because they are programmed to do so by God.) Our final end is the thing for the sake of which every other thing is chosen. All lesser ends are ordered to the final end, which is happiness in God. Only human beings with “well-disposed affections” (615) know this fully, while those who choose sin seek the good in the wrong things.  

Following earlier thinkers like Aristotle and Boethius, Aquinas analyzes various human goals to show that they do not constitute happiness, or the greatest good.

1.  We can see that the greatest good does not consist in wealth and material possessions: The more we possess them the more we see that they are insufficient, because we always want more of them.