Thomas Aquinas (1224-1274)1 is one of the greatest thinkers in Western philosophy and theology. He is even called the angelic Doctor by the Roman Catholic Church. One of his marvelous works is Summa Theologiae which is the primary source of my research to understand the concept of happiness in Summa Theologiae with reference to contemporary psychological studies. Aquinas was a Dominican monk, philosopher, theologian, saint and contemplator, and continues to be important and significant in particular for students of philosophy and theology. Pope Paul VI says that `in order that the students may illumine the mysteries of salvation as completely as possible, they should learn to penetrate them more deeply with the help of speculation, under the guidance of Aquinas, and to perceive their interconnections´. It is also concern of the church regarding studies in schools and universities that by their very constitution individual subjects be pursued according to their own principles, method, and liberty of scientific inquiry, in a way that an ever deeper understanding in these fields may be obtained and that, as questions that are new and current are raised and investigations carefully made according to the example of the doctors of the Church and especially of Aquinas, so that there may be a deeper realization of the harmony of faith and reason.3 Aquinas constructs a vast system of integrating Greek philosophy with the Christian faith in his masterpiece Summa Theologiae. The encounter with the philosophy of Aristotle opened up a new perspective for Aquinas´s synthesis and distinction between philosophy and theology. This was different from the way of the Fathers of the Church because they were confronted by different philosophies of a platonic type in order to get a complete vision of world and of human life including religion; they mainly used Platonism in the light of faith to respond to question of human being whereas Aquinas convincingly explained Aristotelian works. In one of the general audiences pope Benedict XVI explained the relevance of Aquinas who explored the relation of reason and faith in the philosophy of Aristotle and explained it convincingly. A "philosophy" existed that was complete and convincing in itself, a rationality that preceded the faith, followed by "theology", a form of thinking with the faith and in the faith. The pressing question was this: are the world of rationality, philosophy conceived of without Christ, and the world of faith compatible? Or are they mutually exclusive? Elements that affirmed the incompatibility of these two worlds were not lacking, but St Thomas was firmly convinced of their compatibility indeed that philosophy worked out without the knowledge of Christ was awaiting, as it were, the light of Jesus to be complete. This was the great "surprise" of St Thomas that determined the path he took as a thinker. Showing this independence of philosophy and theology and, at the same time, their reciprocal relationality was the historic mission of the great teacher. And thus it can be understood that in the 19th century, when the incompatibility of modern reason and faith was strongly declared, Pope Leo XIII pointed to St Thomas as a guide in the dialogue between them. In his theological work, St Thomas supposes and concretizes this relationality. Faith consolidates, integrates and illumines the heritage of truth that human reason acquires. The trust with which St Thomas endows these two instruments of knowledge faith and reason may be traced back to the conviction that both stem from the one source of all truth, the divine Logos, which is active in both contexts, that of creation and that of redemption. According to Aquinas sacred doctrine is a science. However he says that there are two kinds of sciences. One proceeds from a principle known by the natural light of intelligence, such as arithmetic and geometry and the like. Second kind proceeds from principles known by the light of a higher science. Thus the science of perspective proceeds from principles established by geometry, and music from principles established by arithmetic. Sacred doctrine is a science because it proceeds from principles established by the light of a higher science, the science of God and the blessed. According to Pope Benedict XVI Aquinas presents to us a broad and confident concept of human reason: broad because it is not limited to the spaces of the empirical-scientific reason but open to the whole human being, to the fundamental and inalienable questions of human life; and confident because human reason, especially if it accepts the inspirations of Christian faith, is a promoter of a civilization that recognizes the dignity of the person, the intangibility of rights and the cogency of duties. The most perfect in all nature to be found is a subsistent individual of a rational nature´. Aquinas wrote commentaries on the works of Aristotle to make sense of Aristotle´s philosophy, and not to set out a philosophy of his own. The appreciation of his outstanding value as a philosopher depends on seeing his ostensibly theological works as also fundamentally philosophical and an extrapolation of Aristotle´s view in the light of catholic theology, and from his own contemplation of truth. Under the guidance of his enlightening thoughts, and following his spirit I just try to recreate Aquinas in our times. My major concern is the theory of happiness, and I have restricted myself to explore and understand Aquinas´s view based on Summa Theologiae and psychological studies. Before he begins treatise on human being, he discusses the existence of God, substance of Angels and the work of creation; these three treatises consist of 74 questions, and lay a strong foundation for treatise of human being.