In order to understand the concept of natural law and its principles it is important to understand what Aquinas sought to achieve with his political philosophies. Prior to Aquinas, the mainstream of Christian political thought had been rooted in faith. Faith, by its very definition is the belief in something for which there is no logical or rational justification for. Since the very concept of religion was based on faith, reason was thought to be inimical to faith. Aquinas sought to marry faith and reason and arrive at a political theory that forged a union between the faith of Christian principles and the reason of Greek philosophy.
In the process he rejected one of the central notions of traditional Christian thinking and embraced another. The notion he rejected was one which had been laid down by one of his greatest predecessors, St Augustine; namely, that political societies came into being only as a result of the Fall of Man and were devised as an artificial corrective to sin.
He replaced this principle with the Aristotelian theory that man is by nature a political and social animal. Man, in the eyes of Aquinas therefore was designed by nature to live in communities with others; societies were not simply manifestations of conquest or social contract, they exist because men are naturally members of one another and because men's nature is such that a truly human life is impossible for man unless he lives as a member of a political community.
Aquinas also rejected the Augustinian notion that all terrestrial cities are pervaded by evil, so that Christian hopes for life in a good society must come in the world to come. Aquinas believed that justice could be made to prevail in this world and that it was possible to speak of a...