Whether you realize it or not, you probably adhere to a particular moral theory.
If you formally studied ethics in college or later, chances are you know what your moral theory is. But even if you didn’t, it’s useful to bring your moral theory to full light so that it can be honestly considered and evaluated. That’s part of what critical thinking is for.
Two of the more popular are deontological and teleological theories. “Deontological” theories (from the Greek deon, meaning “ought”) see the essence of the moral life as following absolute commands and avoiding absolute prohibitions—such as, for instance, those believed to come from God. “Teleological” theories (from the Greek telos, meaning “goal”), on the other hand, focus on performing certain actions that best promote achieving the human “goal” of happiness.
But in my experience teaching philosophy, an uncritical relativism is the default stance of most beginning students in both applied ethics and general moral philosophy. ‘Relativism’ is the theory that moral judgments simply express the culturally-developed preferences of individuals, groups, or societies. Moral judgments are thus “relative” to their social and cultural contexts.
Relativism is useful when it comes to the social sciences, as it illuminates the factors that influence people in forming their moral beliefs. Undoubtedly, we understand those influences better now than in the distant past.
At the same time, though, relativism tells us little or nothing about how people actually should behave. It tells us (in part) how and why people come to believe they should behave, but by itself it doesn’t enable us to evaluate those beliefs, and thus change them if rational reflection warrants—and that’s kind of important in human society.
Indeed, the most thoroughgoing moral relativists would say that there is no such thing as how people “should” behave, as distinct from how people deep-down believe they should behave. Moral reflection can at most bring to light and clarify our actual moral beliefs.
But if the task of moral theory is understood to include evaluating and possibly changing some of our moral beliefs, then it’s hard to see how moral relativism qualifies as a moral theory properly so-called. It’s an empirical theory that organizes and explains the data indicating what people’s actual moral beliefs are. That’s about as far as it goes.