Apologetics315 (Terminological Tag):

Ethical Naturalism: is a reductionist view that holds that ethical terms (goodness, worth and right) can be defined by or reduced to natural, scientific properties that are biological, psychological, sociological or physical in nature. For example, according to ethical naturalism the term right in “X is right” means one of the following: “What is approved by most people”; “What most people desire”; “What is approved by an impartial, ideal observer”; “What maximizes desire or interest”; “What furthers human survival.” The important point here is that these moral terms and moral properties are not irreducibly moral in nature. Moral properties (e.g., worth, goodness or rightness) turn out to be properties that are biological or psychological.

Furthermore, according to ethical naturalism, these properties can be measured by science by giving them operational definitions. Consider an example. Suppose “X is right” means “X is what most people desire,” and one goes on to argue that the presence of pleasure and the absence of pain is what most people desire. A scientist could measure the presence of pleasure and the absence of pain by defining such a state in physiological terms — the presence of a certain heart rate, the absence of certain impulses in the nervous system, slight coloration of the skin. “Rightness” means what is desired by most people; what is desired by most people is the presence of pleasure and the absence of pain; and pleasure and pain can be defined by certain physical traits of the body. Thus the moral property of rightness has been reduced to a natural property that can be measured.

Two major objections can be raised against ethical naturalism both based on its moral reductionism. First, it confuses an is with an ought by reducing the latter to the former. Moral properties are normative properties. They carry with them a moral “ought.” If some act has the property of rightness, then one ought to do that act. But natural properties like the ones listed do not carry normativeness. They just are. Second, every attempted reduction of a moral property to a natural one has failed because there are cases where an act is right even if it does not have the natural property, and an act can have the natural property and not be right. For example, suppose one reduces the moral property of rightness in “X is right” to “X is what is approved by most people.” This reduction is inadequate. For one thing, the majority can be wrong. What most people approve of can be morally wrong. If most people approved of torturing babies, then according to this version of ethical naturalism, this act would be right. But even though it was approved by most people, it would still be wrong. On the other hand, some acts can be right even if they are not approved of (or even thought of, for that matter) by most people.[1]

[1] William Lane Craig & J.P. Moreland, Philosophical Foundations for a Christian Worldview (Downers Grove, IL: InterVarsity Press, 2003), p. 401.