George Edward MooreThe naturalistic fallacy is an alleged logical fallacy, described by British philosopher G.E. Moore in Principia Ethica (1903). Moore stated that a naturalistic fallacy was 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.).
The naturalistic fallacy is related to, and often confused with, the is-ought problem (which comes from Hume's Treatise). As a result, the term is sometimes used loosely to describe arguments that claim to draw ethical conclusions from natural facts.
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").
1 Moore's discussion
1.1 The Open Question Argument
2 Other uses
2.1 Appeal to nature
2.2 The is-ought problem
4 See also
5 External links
 Moore's discussion
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, or "survival", in the case of evolutionary ethics). 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:
That "pleased" does not mean "having the sensation of red", or anything else whatever, does not prevent us from understanding what it does mean. It is enough for us to know that "pleased" does mean "having the sensation of pleasure", and though pleasure is absolutely indefinable, though pleasure is pleasure and nothing else whatever, yet we feel no difficulty in saying that we are pleased. The reason is, of course, that when I say "I am pleased", I do not mean that "I" am the same thing as "having pleasure". And similarly no difficulty need be found in my saying that "pleasure is good" and yet not meaning that "pleasure" is the same thing as "good", that pleasure means good, and that good means pleasure. If I were to imagine that when I said "I am pleased", I meant that I was exactly the same thing as "pleased", I should not indeed call that a naturalistic fallacy, although it would be the same fallacy as I have called naturalistic with reference to Ethics.
—G. E. Moore, PE § 12
The point here is connected with Moore's understanding of properties and the terms that stand for them. Moore holds (§7) that properties are either complexes of simple properties, or else irreducibly simple. The meaning of terms that stand for complex properties can be given by using terms for their constituent properties in a definition; simple properties cannot be defined, because they are made up only of themselves and there are no simpler constituents to refer to. Besides "good" and "pleasure", Moore also offers colour terms as an example of indefinable terms; thus if one wants to understand the meaning of "yellow", one has to be shown examples of it; it will do no good to read the dictionary and learn that "yellow" names the colour of egg yolks and ripe lemons, or that "yellow" names the primary colour between green and orange on the spectrum, or that the perception of yellow is stimulated by electromagnetic radiation with a wavelength of between 570 and 590 nanometers. It is true that yellow is all these things, that "egg yolks are yellow" and "the colour perceived when the retina is stimulated by electromagnetic radiation with a wavelength of between 570 and 590 nanometers is yellow" are true statements. But the statements do not give the meaning of the term "yellow", and (Moore argues) to confuse them with a definition of "yellow" would be to commit the same fallacy that is committed when "Pleasure is good" is confused with a definition of "good".
Moore goes on to explain that he pays special attention to the fallacy as it occurs in ethics, and identifies that specific form of the fallacy as ‘naturalistic’, because (1) it is so commonly committed in ethics, and (2) because committing the fallacy in ethics involves confusing a natural object (such as survival or pleasure) with goodness, something that is (he argues) not a natural object. However, it is important to note that in spite of his rhetorical focus on the ‘naturalistic’ nature of the fallacy, Moore was not any more satisfied with theories that attempted to define goodness in terms of non-natural properties than he was with naturalistic theories; indeed, the basis of his criticism of “Metaphysical Ethics” in Chapter IV of Principia Ethica is that theories which define 'good' in terms of supernatural or metaphysical properties rest on the very same fallacy as naturalistic theories (§69). The target of Moore's discussion of the "naturalistic fallacy" is reductionism at least as much as it is naturalism specifically, and the important lesson, for Moore, is that the meaning of the term "good" and the nature of the property goodness are irreducibly sui generis.
 The Open Question Argument
Moore's argument for the undefinability of “good” (and thus for the fallaciousness of the “naturalistic fallacy”) is often called the Open Question Argument; it is presented in §13 of Principia Ethica. The argument hinges on the nature of statements such as "Anything that is pleasant is also good" and the possibility of asking questions such as "Is it good that x is pleasant?" According to Moore, these questions are open and these statements are significant; and they will remain so no matter what is substituted for "pleasure". Moore concludes from this that any analysis of value is bound to fail. In other words, if value could be analyzed, then such questions and statements would be trivial and obvious. Since they are anything but trivial and obvious, value must be indefinable. Critics of Moore's arguments sometimes claim that he is appealing to general puzzles concerning analysis (cf. the paradox of analysis), rather than revealing anything special about value. Other responses appeal to the Fregean distinction between sense and reference, allowing that value concepts are special and sui generis, but insisting that value properties are nothing but natural properties (this strategy is similar to that taken by non-reductive materialists in philosophy of mind).
An important response to the open question argument by contemporary ethical naturalists (e.g., Peter Railton) is to understand a claim like "The Good is pleasure" as an a posteriori identity claim on a par with "Water is H2O". The question "This is H2O but is it water?" is intelligible and so, in that limited sense, whether or not water is H2O is an open question. But that does not lead us to conclude that water is not H2O. "Water is H2O" is an identity claim that is known to be true a posteriori (i.e., it was discovered via empirical investigation). The fact that this truth is not known merely by conceptually analyzing the term "water" (and the corresponding fact that the aforementioned open question is at least intelligible) does not falsify the identity claim. Similarly, an ethical naturalist might argue that, say, "The Good is pleasure" is an a posteriori identity claim whose truth is discovered empirically. That we can intelligibly ask "I see that this is pleasant, but is it good?" simply means that we cannot conceptually analyze "good" in terms of "pleasure". It does not mean that goodness is not the same thing as pleasure. "Good" and "pleasant" might pick out (refer to) the same thing. Whether or not this is the case is a matter of empirical investigation, and not conceptual analysis, according to this kind of ethical naturalist.
 Other uses
 Appeal to nature
Some people use the phrase "naturalistic fallacy" or "Appeal to nature" to characterise inferences of the form "This behaviour is natural; therefore, this behaviour is morally acceptable" or "This behaviour is unnatural; therefore, this behaviour is morally unacceptable". Such inferences are common in discussions of homosexuality and cloning, to take two examples. While such inferences may indeed be fallacious, it is important to realise that Moore is not concerned with them. He is instead concerned with the semantic and metaphysical underpinnings of ethics.
 The is-ought problem
The term "naturalistic fallacy" is also sometimes used to describe the deduction of an "ought" from an "is" (the Is-ought problem), and has inspired the use of mutually reinforcing terminology which describes the converse (deducing an "is" from an "ought") either as the "reverse naturalistic fallacy" or the "moralistic fallacy". An example of a naturalistic fallacy in this sense would be to conclude Social Darwinism from the theory of evolution by natural selection, and of the reverse naturalistic fallacy to argue that the immorality of survival of the fittest implies the theory of evolution is false.