There are two basic types of ethical theories. One kind of ethical theory is descriptive, and these theories attempt to explain why people judge things to be good or bad. The second kind of ethical theory is normative (or prescriptive if you prefer), meaning that these theories attempt to argue for a system of ethics that provides rules that you should follow, even if not all people do in fact follow them. For example if you thought that theft was bad a descriptive ethical theory would explain why you thought it as bad while a normative theory would tell you if you should think that it is bad.
It could be argued that descriptive ethical theories are not really about ethics but about how ethical terms are used, although this does not necessarily make them bad theories, just not ethical ones. I personally favor Charles Stevenson’s ethical theory, described in The Emotive Meaning of Ethical Terms, as an excellent descriptive theory. Stevenson’s theory argues that good and bad are simply tools we use to persuade other people for or against certain actions. Under this theory my labeling of theft as bad does not indicate that there is some property of the action that I am pointing out, but that I do not want people to commit theft. However if we had a theory about why people classified objects as red or blue we would not consider it to be a theory about color, we would only consider a theory that explained the origin of our color perceptions to be a theory about color.
Likewise a theory such as Stevenson’s may agree with experience, but it seems to be leaving out something essential about the nature of ethics. Intuitively we feel that there really is such a thing as right or wrong independent of what people think, and that ethics should explain it, not our perceptions of it. Perhaps Stevenson’s analysis is accurate we might admit, but doesn’t my desire to persuade people not to steal arise because I see that the action is wrong? We also might think that an ethical theory should be able to help guide our actions when confronted with difficult ethical dilemmas, such as the morality of euthanasia. A descriptive theory cannot help here either, because it does not inform us as to what we should do, only why others might label our actions as good or bad later.
If we want to create a normative (i.e. reason-giving) ethical system however we must first address the issue of why it is reason giving. Simply labeling a system as ethical is not sufficient reason to be guided the rules it proposes. In order to show why it is reason giving ethical action then is often identified as a means to some ends that we desire. For example the ancients believed that virtuous action lead to a happier life. Kant on the other hand argued that ethical action was rational action, and thus your desire for rationality would give you cause to be ethical. Finally, some think that human beings have an innate ethical sense, as evidenced by ethical intuitions, and that obeying an ethical system will allow you to live in minimal conflict with these intuitions. Whatever the basis for ethics is decided to be the rules can then be deduced from this basis, for example Kant’s deduction of the categorical imperative from what can be rationally willed and Plato’s defense of justice.
Of course I haven’t defended normative ethics from the assertion that there is no such thing as real, objective, right and wrong. Those who hold such a view would conclude that descriptive theories are all that we can hope for. However the defense of ethics against such a position must be left for another day, and a much longer essay.