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Natural Law Theory

I. Natural law

St. Thomas Aquinas is associated with the theory of Natural law. He derived this theory by building on the ethics of Aristotle, and in particular on Aristotle’s attempt to justify that ethics in his function argument. This function argument claims to derive man’s purpose from his function – specifically, it claims that from the premise that man is a rational animal it follows that man should live the life of reason. Whereas Aristotle tends to see the function of man as a biological fact, Aquinas argues that everything has been created by God for a specific purpose, and that to fulfill this purpose is to lead the good life. Moral laws are derived by Aquinas from Natural Law.
He claims that each man has been created by God for a specific purpose. Following Aristotle he links happiness to virtuous behaviour. Likewise, he follows Aristotle in claiming that the virtuous life is the life of reason. He adds the Christian concept of sin to this, arguing that sin is falling short of the ideal life of reason. Sin is effectively “missing the mark”.
In modern times Natural Law theory has been used to criticize liberal sexual morality. It is claimed that the natural purpose of sex is to procreate – create babies, and, therefore, it is sinful to masturbate, to be homosexual, to practice anal or oral sex, and to use contraceptives during sex.

II. Critique

Natural law theory is based on the work of St. Thomas Aquinas. Whilst the element of purpose is disguised in Aristotle’s function argument, Aquinas has the virtue of making it more explicit. He argues that man’s function is his purpose given to him by God. However, Aquinas’s argument still commits the fact/value fallacy – as does Aristotle’s argument. It still claims to be able to read from facts about human nature to facts about God’s purpose for human beings, to values about how we ought to live. The whole reputed force of the argument derives from the idea that the purpose of a man can be read from certain biological facts. However many steps are introduced into the inference it is still true that from a mere biological fact, no value can be derived. For example, from the biological fact that “sex produces babies” it does not follow that sex is only morally justified if it is for producing babies; nor does it follow that God only intends sex to be used for producing babies, which is the additional step in the argument that Aquinas effectively introduces.
This kind of reasoning is also selective about what facts it will take note of. For example (and if you are lucky!) sex also produces pleasure (fact) and causes bonding between sexual partners (fact). Whilst neither of these facts entail a moral conclusion about sex, they chime better with a liberal interpretation of the value of sex – for if sex causes partners to bond, and if it is morally good for partners to bond (major premise about values required to infer a further moral conclusion), then it follows that sex is a good thing per se, and likewise, there is no prohibition against the kind of sex (oral, anal, use of contraceptives) involved.
It is a common human failing to be prejudiced in favour of values that we hold for reasons that are altogether (or almost altogether) unanalysable. For example, a person brought up with a certain background will feel a strong prejudice against certain sexual practices. The sentiment is rooted in forces that are effectively not capable of examination. Thus, we are all inclined to distort our observations to favour our moral bias. Our perceptions are “contaminated” by our “emotions”. Natural law theory is particularly vulnerable to this form of contamination, as the above example illustrates; the perceptions are distorted since facts that cast a favourable light on different sexual practices are conveniently ignored.
However, this is not to say that any particular sexual practice is advocated in this article. That is left to each person’s conscience! The purpose here is only to expose fallacious thinking, and Aquinas, as Aristotle before him, does commit the fact/value fallacy.