The naturalistic fallacy is often claimed to be a formal fallacy. It was described and named by British philosopher G. E. Moore in his 1903 book Principia Ethica. Moore stated that a naturalistic fallacy is committed whenever a philosopher attempts to prove a claim about ethics by appealing to a definition of the term "good" in terms of one or more natural properties (such as "pleasant", "more evolved", "desired", etc.). Arthur N. Prior defined it as
…the assumption that because some quality or combination of qualities invariably and necessarily accompanies the quality of goodness, or is invariably and necessarily accompanied by it, or both, this quality or combination of qualities is identical with goodness. If, for example, it is believed that whatever is pleasant is and must be good, or that whatever is good is and must be pleasant, or both, it is committing the naturalistic fallacy to infer from this that goodness and pleasantness are one and the same quality. The naturalistic fallacy is the assumption that because the words 'good' and, say, 'pleasant' necessarily describe the same objects, they must attribute the same quality to them. 
The naturalistic fallacy is related to (and even confused with) the is-ought problem, which comes from Hume's Treatise. The term is sometimes used loosely to describe arguments that claim to draw ethical conclusions from natural facts. This view is in opposition to some forms of ethical naturalism.
Alternatively, the phrase "naturalistic fallacy" is used to refer to the claim that what is natural is inherently good or right, and that what is unnatural is bad or wrong (see "Appeal to nature"). It is the converse of the moralistic fallacy, the notion that what is good or right is natural and inherent.
Moore's argument in Principia Ethica is (among other things) a defense of ethical non-naturalism; he argues that the term "good" (in the sense of intrinsic value) is indefinable, because it names a simple, non-natural property. It is, rather, "one of those innumerable objects of thought which are themselves incapable of definition, because they are the ultimate terms by reference to which whatever is capable of definition must be defined" (Principia Ethica § 10 ¶ 1). By contrast, many ethical philosophers have tried to prove some of their claims about ethics by appealing to an analysis of the meaning of the term "good"; they held, that is, that "good" can be defined in terms of one or more natural properties which we already understand (such as "pleasure", in the case of hedonists). Moore coined the term "naturalistic fallacy" to describe arguments of this form; he explains (in § 12) that the fallacy involved is an instance of a more general type of fallacy, which he leaves unnamed, but which we might call the "definitional fallacy". The fallacy is committed whenever a statement to the effect that some object has a simple indefinable property is misunderstood as a definition that gives the meaning of the simple indefinable property:
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