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In philosophical ethics, the term "naturalistic fallacy" was introduced by British philosopher G. E. Moore in his 1903 book Principia Ethica. Moore argues it would be fallacious to explain that which is good reductively, in terms of natural properties such as "pleasant" or "desirable".
The naturalistic fallacy is close to but not identical with the fallacious appeal to nature, the claim that what is natural is inherently good or right, and that what is unnatural is inherently bad or wrong. The fallacious appeal to nature would be the reverse of a moralistic fallacy: that what is good or right is thus natural.
Furthermore, Moore's naturalistic fallacy is very close to (and even confused with) the is–ought problem, which comes from Hume's Treatise. However, unlike Hume's view of the is–ought problem, Moore (and other proponents of ethical non-naturalism) did not consider the naturalistic fallacy to be at odds with moral realism.
Different common uses 
The is–ought problem 
In using his categorical imperative Kant deduced that experience was necessary for their application. But experience on its own or the imperative on its own could not possibly identify an act as being moral or immoral. We can have no certain knowledge of morality from them, being incapable of deducing how things ought to be from the fact that they happen to be arranged in a particular manner in experience.
Bentham, in discussing the relations of law and morality, found that when people discuss problems and issues they talk about how they wish it would be as opposed to how it actually is. This can be seen in discussions of natural law and positive law. Bentham criticized natural law theory because in his view it was a naturalistic fallacy, claiming that it described how things ought to be instead of how things are.
Moore's discussion 
According to G. E. Moore's Principia Ethica, when philosophers try to define "good" reductively in terms of natural properties like "pleasant" or "desirable", they are committing the naturalistic fallacy.
...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.—Arthur N. Prior, Logic And The Basis Of Ethics
In defense of ethical non-naturalism, Moore's argument is concerned with the semantic and metaphysical underpinnings of ethics. In general, opponents of ethical naturalism reject ethical conclusions drawn from natural facts.
Moore argues that good, in the sense of intrinsic value, is simply ineffable: it cannot be defined because it is not a natural property, being "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". On the other hand, ethical naturalists eschew such principles in favor of a more empirically accessible analysis of what it means to be good: for example, in terms of pleasure in the context of hedonism.
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, Principia Ethica § 12
In §7, Moore argues that a property is either a complex of simple properties, or else it is irreducibly simple. Complex properties can be defined in terms of their constituent parts but a simple property has no parts. In addition to "good" and "pleasure", Moore suggests that colour qualia are undefined: if one wants to understand yellow, one must see 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, because yellow is all that and more, by the open question argument.
Appeal to nature 
Some people use the phrase "naturalistic fallacy" or "appeal to nature" to characterize inferences of the form "This behaviour is natural; therefore, this behaviour is morally acceptable" or "This property is unnatural; therefore, this property is undesireable." Such inferences are common in discussions of homosexuality, environmentalism and veganism.
The naturalistic fallacy is the idea that what is found in nature is good. It was the basis for Social Darwinism, the belief that helping the poor and sick would get in the way of evolution, which depends on the survival of the fittest. Today, biologists denounce the Naturalistic Fallacy because they want to describe the natural world honestly, without people deriving morals about how we ought to behave (as in: If birds and beasts engage in adultery, infanticide, cannibalism, it must be OK). The moralistic fallacy is that what is good is found in nature. It lies behind the bad science in nature-documentary voiceovers: lions are mercy-killers of the weak and sick, mice feel no pain when cats eat them, dung beetles recycle dung to benefit the ecosystem and so on. It also lies behind the romantic belief that humans cannot harbor desires to kill, rape, lie, or steal because that would be too depressing or reactionary.
Some philosophers reject the naturalistic fallacy and/or suggest solutions for the proposed is-ought problem.
Sam Harris argues that it is possible to derive "ought" from "is", and even that it has already been done to some extent.  He sees morality as a budding science. This view is critical of Moore's "simple indefinable terms" (which amount to qualia), arguing instead that such terms actually can be broken down into constituents.
Ralph McInerny suggests that "ought" is already bound up in "is", in so far as the very nature of things have ends/goals within them. For example, a clock is a device used to keep time. When one understands the function of a clock, then a standard of evaluation is implicit in the very description of the clock, i.e., because it "is" a clock, it "ought" to keep the time. Thus, if one cannot pick a good clock from a bad clock, then one does not really know what a clock is. In like manner, if one cannot determine good human action from bad, then one does not really know what the human person is.
See also 
- Appeal to nature
- Appeal to novelty
- Appeal to tradition
- Definist fallacy
- Evolution of morality
- Fact-value distinction
- Moralistic fallacy
- Philosophical naturalism
- Norm (philosophy)
- Open Question Argument
- Value theory
Further reading 
- Frankena, W. K. (1939). "The Naturalistic Fallacy". Mind. XLVIII (192): 464–77. doi:10.1093/mind/XLVIII.192.464. JSTOR 2250706.
- Curry, Oliver (2006). "Who's afraid of the naturalistic fallacy?". Evolutionary Psychology 4: 234–47.
- Walter, Alex (2006). "The anti-naturalistic fallacy: Evolutionary moral psychology and the insistence of brute facts". Evolutionary Psychology 4: 33–48.
- Wilson, David Sloan; Dietrich, Eric; Clark, Anne B. (2003). Biology and Philosophy 18 (5): 669–81. doi:10.1023/A:1026380825208.
- Moore, G.E. Principia Ethica § 10 ¶ 3
- Prior, Arthur N. (1949), Chapter 1 of Logic And The Basis Of Ethics, Oxford University Press (ISBN 0-19-824157-7)
- Moore, G.E. Principia Ethica § 10 ¶ 1
- Williams, Bernard Arthur Owen (2006). Ethics and the Limits of Philosophy. Abingdon, Oxfordshire: Taylor & Francis. p. 121. ISBN 978-0-415-39984-5.
- Q&A: Steven Pinker of 'Blank Slate', United Press International, 10/30/2002, http://pinker.wjh.harvard.edu/books/tbs/media_articles/2002_10_30_upi.html
- Harris, Sam (29 March 2010). "Moral Confusion in the Name of "Science"". huffingtonpost.com. Retrieved 30 March 2011.
- Harris, Sam (22 March 2010). "Science Can Answer Moral Questions". ted.com. Retrieved 30 March 2011.
- McInerny, Ralph (1982). Ethica Thomistica. Cua Press. p. Chp. 3.
- Moore, George Edward (1903). Principia Ethica. Cambridge: Cambridge University Press. ISBN 0-334-04040-X.
- Appeal to Nature entry in The Fallacy Files
- Principia Ethica
- G.E. Moore entry in the Stanford Encyclopedia of Philosophy
- Moral non-naturalism entry in the Stanford Encyclopedia of Philosophy