Another tradition, which has long tried to establish a single, clear Christian ethic, is natural moral law. To understand the system we must look back to the work of Aristotle.
For Aristotle all things can be understood in four ways, or through four "causes" as he put it. Everything has material causes, ingredients, efficient causes, agents which bring together the ingredients, a formal cause, a shape or recipe, and a final cause, a purpose or reason for being. Taking the example of an oak tree - the material causes are carbon, hydrogen, oxygen and so on, the efficient causes are acorn, sun, rain etc. The formal cause is having a trunk, branches and knobbly leaves and the final cause is to live, grow and eventually reproduce. For Aristotle human beings are the same. We have material causes - carbon, oxygen, sulphur and so on. We have efficient causes - parents, food, shelter etc. We have a formal cause (two arms, two legs - but more freedom and the potential to make moral choices) and we have a final cause, to live, learn, procreate and pass on our learning in a peaceful society. For Aristotle good things fulfil their final causes - good actions contribute to things doing so, they enable things and people to flourish. Bad things are destructive and prevent that.
Aquinas - what is a ‘good' life?
Aquinas developed the Aristotelean principle and concluded that the primary precepts of natural law (the common characteristics of a good life) were to live peacefully, to be reasonably prosperous, to procreate, acquire wisdom through philosophy and pass on wisdom and express gratitude through praise. He argued that all actions which generally contributed to the primary precepts are good and those which generally take away from the primary precepts are bad. These are the secondary precepts of natural law - not to murder, steal, lie etc. The primary precepts also suggest that some forms of behaviour are virtuous and others full of vice. Virtues tend, for Aquinas, to occupy the middle ground between extremes of vice. For example it is virtuous to be courageous but vice lies in both being rash and in being cowardly.
According to Aquinas, people do not tend to do something that they know consciously to be wrong. Most evil is done through the pursuit of apparent goods - that is, we might steal to feed our family under the impression that satisfying the hunger of children is a greater duty than obeying the law and respecting the property of others. He argued that the intention is important, as well as the action itself and its consequences, though clearly the intention may be difficult to establish. Today Catholic ethics are still broadly Thomist in character. Though there are many neo-naturalist theories of ethics (described under Natural law in this section) Catholics still tend to accept St Thomas' Primary Precepts and Virtues as the basis of their ethic.
In conclusion then it is clear that while there is diversity within Christian ethics, there is also a thread of unity. The vast majority of Christians hold to the sanctity of life, the need for human beings to flourish and to act with love. The differences come in the definition of what is a human person, how human beings best flourish and what the principle of love really is. None of these issues are easy to resolve, however Christians would be more likely to come together if they addressed the central philosophical issues directly in open debate, rather than focusing on their different opinions on applied issues.