|This is not Christian ethics|
When St. Thomas Aquinas turns to his study of the moral life, the return of the rational creature to his Creator, he begins not with the personal conscience of the believer, nor with the objective precepts of the moral law, nor even with virtue and vice, but with the pursuit of happiness. In the Summa Theologica, St. Thomas tells us that happiness is the last end of all men, it is the one end two which all men tend. This, then, is the goal of morality: To aid us in the attainment of happiness, which is found in the vision of the Divine Essence.
In this matter, St. Thomas is following his divine Teacher, who began the Sermon on the Mount, the great discourse on the Christian life, with the Beatitudes:
He began to teach them saying: Blessed are they…
Christ our Savior does not begin his instruction with law, nor with the subjective discernment of the individual conscience, but with beatitude. Blessed, he says, which means happy. He draws the disciples to the Christian life through appealing to their deepest desire – he who made us knows that we want to be happy, and he also knows how we will get there.