Naturalism, sometimes also called definism, is a theory in meta-ethics that holds that ethical terms can be defined; the meaning of ethical sentences can be given in totally non-ethical terms. So to the question, "Can the meaning of ethical sentences be restated in other words that do not use normative concepts like 'good' and 'right'?" the naturalist answers, "Definitely." On the naturalist's view, ultimately, goodness and right are natural properties--they are ultimately properties of things that can be located in the natural world.

We might give a more detailed definition, in terms of propositions and reduction, that, in generalities, expresses the general understanding of the term:

Naturalism is the view that ethical sentences express propositions and that they can be reduced to nonethical sentences.

Examination of definition

Let us take a closer look at each part of this definition. First, there is the notion that ethical sentences express propositions. The latter view, that ethical sentences express propositions, is called cognitivism: see cognitivism and non-cognitivism.

Ethical naturalism combines cognitivism with moral reductionism: as the second part of the above definition says, ethical sentences "can be reduced to nonethical sentences." To say that ethical sentences can be reduced to nonethical sentences is to say that you can state the meaning of ethical sentences in other words, in sentences that do not include any ethical terms like 'good' and 'right'. (See reduction.) In other words, all talk of good and bad, right and wrong, moral and immoral, and so forth, is (ultimately) just shorthand for some complex propositions about what human beings need, or desire, or what gives them pleasure, or what secures their long-term happiness, and so forth. So the notion that ethical sentences can be reduced to nonethical sentences really amounts to saying that ethical sentences are a kind of shorthand, a kind of useful abbreviation, for claims about what are ultimately nonethical facts about human needs, desires, and so forth.

Theory of Value

The theory of value--an important branch of ethics--contains a number of theories of what 'good' means, or (construed differently) of what general sorts of things are good. Hedonism, just to take one example, is the view that goodness is ultimately just pleasure; hedonism is one attempt at giving a theory of value. Therefore, one could look at the theory of value as a way of thinking ethical naturalism through; the theory of value can be regarded as an attempt to figure out how to reduce goodness to nonethical properties. So, if it is initially puzzling how one could ever reduce ethical sentences to nonethical sentences, one need only review the theory of value for many examples of such reductions. We should add, however, that not all philosophers working on value theory would view their theories as reductions.