Part I History of Ethics
Life of Socrates
Part II Concepts and Problems
Normative Ethics and Applied Ethics
Part III Applied Ethics
Field of Applied Ethics
St. Thomas Aquinas
In the 13th Century, Aristotle's works were 'rediscovered' in the West and translated into Latin. These translations of 'The Philosopher' (as Aquinas called him) became an integral part of some of Aquinas' most important writings. (See the Jacques Maritain site at the University of Notre Dame for an overview of Thomas Aquinas' life and work.)
That part of Eternal Law that pertains to the behavior of human beings is the proper domain of Natural Law. Vernon Bourke, quoting from Aquinas, describes it thus:
'Good is to be done and promoted and evil to be avoided' (ST I-II, 94, 2). Since this rule does not specify what is good, it cannot be further analyzed to find more specific moral rules. It is a principle formally governing practical reasoning and in this sense Thomas calls it the first precept of natural law. To determine what are the proximate natural goods for man, Aquinas suggests that reason naturally apprehends as goods those objects that satisfy man's basic inclinations. On the lowest level are those physical goods that all beings incline to, such as self-preservation. Second are biological goods that men tend towards, as do all living things: the procreation and care of offspring, for instance. In the third and highest place he puts those values that satisfy man as a rational being: the knowledge of truth about God and the advantage of living in the society of other humans.Human Law involves those civil laws that govern communities. These civil laws may indeed vary from town to town as long as they don't violate the precepts of Natural Law.
Finally, Divine Law pertains to God's special plans for humanity and is revealed through, for example, sacred scripture.
Examples of these 'laws' could be (1) the law of gravity as governing the motion of physical objects, (2) prohibition of artificial birth control as violating our natural tendency toward procreation, (3) laws regulating the traffic in a particular city and disobedience with regard to laws that seek to destroy religious faith (through, for example, the banning of Mass), (4) knowledge, through God's Grace, of our supernatural rewards (as revealed in the New Testament).
See Vernon Bourke for a discussion of Aquinas' basic ethical theory. See also excerpts from the Routledge Encyclopedia of Philosophy article on Aquinas. For an example of how Aquinas appropriates Aristotelian moral philosophy, see his discussion of "virtue as a mean" in Summa Theologica.