In metaethics, the naturalistic fallacy is the claim that it is possible to define good in terms of merely described entities, properties, or processes such as pleasant, desirable, or fitness.^[1] The term was introduced by British philosopher G. E. Moore in his 1903 book Principia Ethica.^[2]

Moore's naturalistic fallacy is closely related to the is–ought problem, which comes from David Hume's Treatise of Human Nature (1738–40); 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.

The term naturalistic fallacy is sometimes used to label the problematic inference of an ought from an is (the is–ought problem).^[3] Michael Ridge relevantly elaborates that "[t]he intuitive idea is that evaluative conclusions require at least one evaluative premise—purely factual premises about the naturalistic features of things do not entail or even support evaluative conclusions."^[1] This problematic inference usually takes the form of saying that if people generally do something (e.g., eat three times a day, smoke cigarettes, dress warmly in cold weather), then people ought to do that thing. The naturalistic fallacy occurs when the is–ought inference ("People eat three times a day, so it is morally good for people to eat three times a day") is justified by the claim that whatever practice exists is a natural one ("because eating three times a day is pleasant and desirable").

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, instead of 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 an instance of the naturalistic fallacy, claiming that it described how things are rather than how they ought to be.

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.^[4]

— Arthur N. Prior, Logic And The Basis Of Ethics

In defense of ethical non-naturalism against ethical naturalism, Moore's argument is concerned with the semantic and metaphysical underpinnings of ethics. Moore argues that good, in the sense of intrinsic value, is simply ineffable. It cannot be defined because it is not reducible to other properties, 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".^[5] 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 lacks 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.

Some people use the phrase, naturalistic fallacy or appeal to nature, in a different sense, to characterize inferences of the form "Something is natural; therefore, it is morally acceptable" or "This property is unnatural; therefore, this property is undesirable." Such inferences are common in discussions of medicine, 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).

— Steven Pinker^[6]

Bernard Williams called Moore's use of the term naturalistic fallacy a "spectacular misnomer", the matter in question being metaphysical, as opposed to rational.^[7]

Some philosophers reject the naturalistic fallacy or suggest solutions for the proposed is–ought problem.

Ralph McInerny suggests that ought is already bound up in is, insofar 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.^{[8][page needed]}

The belief that naturalistic fallacy is inherently flawed has been criticized as lacking rational bases, and labelled anti-naturalistic fallacy.^{[9][page needed]} For instance, Alex Walter wrote:

"The naturalistic fallacy and Hume's 'law' are frequently appealed to for the purpose of drawing limits around the scope of scientific inquiry into ethics and morality. These two objections are shown to be without force."^[10]

That is because said beliefs implicitly assert that there is no connection between the facts and the norms (in particular, between the facts and the mental process that led to adoption of the norms).^[11] However, philosophers show that these connections are inevitable.

A very basic example is that if people view rescuing people as morally correct, this would shape their beliefs on what constitutes danger and what situations warrant intervention. For wider-ranging examples, if one believes that a certain ethnic group of humans have a population-level statistical hereditary predisposition to destroy civilization while the other person does not believe that such is the case, that difference in beliefs about factual matters will make the first person conclude that persecution of said ethnic group is an excusable "necessary evil" while the second person will conclude that it is a totally unjustifiable evil.

Similarly, if two people think it is evil to keep people working extremely hard in extreme poverty, they will draw different conclusions on de facto rights (as opposed to purely semantic rights) of property owners. The latter is dependent on whether they believe property owners are responsible for the aforementioned exploitation. One who accepts this premise would conclude that it is necessary to persecute property owners to mitigate exploitation. The one who does not, on the other hand, would conclude that the persecution is unnecessary and evil.^[12][13]

Some critics of the assumption that is-ought conclusions are fallacies point at observations of people who purport to consider such conclusions as fallacies do not do so consistently. Examples mentioned are that evolutionary psychologists who gripe about "the naturalistic fallacy" do make is-ought conclusions themselves when, for instance, alleging that the notion of the blank slate would lead to totalitarian social engineering or that certain views on sexuality would lead to attempts to convert homosexuals to heterosexuals. Critics point at this as a sign that charges of the naturalistic fallacy are inconsistent rhetorical tactics rather than detection of a fallacy.^[14][15]

A criticism of the concept of the naturalistic fallacy is that while "descriptive" statements (used here in the broad sense about statements that purport to be about facts regardless of whether they are true or false, used simply as opposed to normative statements) about specific differences in effects can be inverted depending on values (such as the statement "people X are predisposed to eating babies" being normative against group X only in the context of protecting children while the statement "individual or group X is predisposed to emit greenhouse gases" is normative against individual/group X only in the context of protecting the environment), the statement "individual/group X is predisposed to harm whatever values others have" is universally normative against individual/group X. This refers to individual/group X being "descriptively" alleged to detect what other entities capable of valuing are protecting and then destroying it without individual/group X having any values of its own. For example, in the context of one philosophy advocating child protection considering eating babies the worst evil and advocating industries that emit greenhouse gases to finance a safe short term environment for children while another philosophy considers long term damage to the environment the worst evil and advocates eating babies to reduce overpopulation and with it consumption that emits greenhouse gases, such an individual/group X could be alleged to advocate both eating babies and building autonomous industries to maximize greenhouse gas emissions, making the two otherwise enemy philosophies become allies against individual/group X as a "common enemy". The principle, that of allegations of an individual or group being predisposed to adapt their harm to damage any values including combined harm of apparently opposite values inevitably making normative implications regardless of which the specific values are, is argued to extend to any other situations with any other values as well due to the allegation being of the individual or group adapting their destruction to different values. This is mentioned as an example of at least one type of "descriptive" allegation being bound to make universally normative implications, as well as the allegation not being scientifically self-correcting due to individual or group X being alleged to manipulate others to support their alleged all-destructive agenda which dismisses any scientific criticism of the allegation as "part of the agenda that destroys everything", and that the objection that some values may condemn some specific ways to persecute individual/group X is irrelevant since different values would also have various ways to do things against individuals or groups that they would consider acceptable to do. This is pointed out as a falsifying counterexample to the claim that "no descriptive statement can in itself become normative".^[16][17]

• Moore, George Edward (1903). Principia Ethica. Cambridge: Cambridge University Press. ISBN 0-334-04040-X.

• 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. doi:10.1177/147470490600400120.
• Walter, Alex (2006). "The anti-naturalistic fallacy: Evolutionary moral psychology and the insistence of brute facts". Evolutionary Psychology. 4: 33–48. doi:10.1177/147470490600400102.
• Wilson, David Sloan; Dietrich, Eric; Clark, Anne B. (2003). "On the inappropriate use of the naturalistic fallacy in evolutionary psychology". Biology and Philosophy. 18 (5): 669–81. doi:10.1023/A:1026380825208. S2CID 30891026.

• Principia Ethica Archived 2021-04-12 at the Wayback Machine
• Zalta, Edward N. (ed.). "G.E. Moore". Stanford Encyclopedia of Philosophy.
• Zalta, Edward N. (ed.). "Moral non-naturalism". Stanford Encyclopedia of Philosophy.
• Appeal to Nature entry in The Fallacy Files