MODERN MORALITY (I know, I know, doesn't that sound like an oxymoron?) is distinctly a forward-looking system, that is, it attempts to take basic moral principles and apply them to complex circumstances. This process is called casuistry, which is kind of a dirty word, but it still is a valid endeavor. This ought to be contrasted with the traditional, backwards-looking approach to morality, which seeks to find basic moral principles, and emphasizes the study of famous texts from throughout history.
I am reminded of this distinction because of this week's visit by Professor Peter Singer of Princeton, to Washington University in Saint Louis. Singer is perhaps the most famous, or perhaps most notorious philosopher of bioethics working today, being one of the intellectual founders of the contemporary animal rights movement. His work is seen to justify abortion, eugenics, euthanasia, and other things I don't care to mention. See this article by Steve Skojec which covers some of his ideas.
Singer's conclusions are based on the theory of Utilitarianism, which is a kind of quantitative and reductionistic calculus of morality. As a kind of moral mathematics, a utilitarian can plug in moral principles and the technique spits out various conclusions, which of course are highly influenced by basic moral assumptions. Typically, 'happiness' is the variable that the mathematics attempts to maximize, but the varying definitions of happiness leads to various schools of Utilitarian thought.
Being one of the cornerstone ideas of the Enlightenment, both the Left and the Right make great use of this theory. If we assume that money equals happiness, and that a billion dollars will make you a thousand times happier than a million dollars, then Utilitarianism equals classical economic theory. But if we assume that pleasure equals happiness ('hedonism'), and that short-term pleasures are more important than long term pleasure, then the consequences of Utilitarianism include Planned Parenthood and sex education in Kindergarden. Aquinas has a few things to say about both definitions of happiness.
The founders of Utilitarianism appeared on the Index librorum prohibitorum, or index of forbidden books, as being harmful to the faith and morals of the Catholic laity. The reason for this is because the theory, at its core, is essentially that "the ends justify the means", which is the creed of tyrants.
As far as I know, Prof. Singer is a decent enough chap, and we both have something in common. Some of his Jewish relatives, and some of my Catholic relatives, were killed together by the Neo-Pagan Nazis in Łódź, Poland, because of their Lebensunwertes Leben — life unworthy of life.