The naturalistic fallacy is an alleged fallacy of moral reasoning. The British philosopher George Edward Moore (1873-1958) introduces the naturalistic fallacy in his seminal work Principia Ethica (1903). Although the naturalistic fallacy began with Moore, it has been revised over the years in a sub-field of ethics known as “metaethics,” which is the study of the language, metaphysics, and epistemology of ethics.
The naturalistic fallacy is the fallacy of attempting to define evaluative concepts with descriptive concepts (Pence 2000, 37). The naturalistic fallacy is related to but is not identical with the “is-ought fallacy,” which is the fallacy of drawing evaluative conclusions from descriptive premises.
The Scottish philosopher David Hume (1711-1776) discovered the is-ought fallacy and reported it in the volume labeled "Of Moral"’ in his classic text A Treatise of Hume Nature in 1739. An example of the is-ought fallacy is concluding that gay marriage ought to be illegal in America because there is a consensus among the American people that gay marriage ought to be illegal. The fallacy here should be obvious since an analogous argument could show that interracial marriage should have been illegal in Alabama in 1999 before Alabama voters repealed the century-old ban on interracial marriages in 2000. The fact that there is a consensus does not justify a claim of how something should be.
The is-ought fallacy, as Hume would put it, lies in the logical gap between ought-statements and is-statements. Is-statements (also known as “descriptions”) are claims about what there is (e.g., sometimes people lie). Ought-statements (also known as “prescriptions” or “evaluations”) are claims about what should be (e.g., people should always tell the truth). Is-statements are exemplified in the sciences, whereas ought-statements are exemplified in ethics and aesthetics. Hume claims that inferring ought-statements from is-statements is deductively invalid. Thus no amount of descriptive facts force evaluative claims onto us. This means that we can always have evaluative reasons for interpreting descriptive facts in one way versus another.
In order to get around Hume’s is-ought fallacy, some eighteenth- and nineteenth-century moral philosophers simply defined goodness in terms of natural properties, much like scientists define natural things in terms of natural properties (e.g., electricity is moving charge). The utilitarians Jeremy Bentham (1748-1832) and John Stuart Mill (1806-1873) are prime examples of such “naturalistic” philosophers. Actually, in metaethics a philosopher who believes that moral concepts can be defined with natural concepts is called a "moral naturalist" and a supporter of "moral naturalism."
These early utilitarians believed that the moral term "good" can be defined entirely by the phrase "whatever produces the greatest amount of pleasure for the greatest number of sentient beings," although Bentham and Mill disagreed about what sort of pleasure should be emphasized.
G.E. Moore claimed that the mere act of defining moral concepts with natural concepts was a fallacy, the naturalistic fallacy. Moore used the phenomenal quality (sometimes called "qualia") "yellow" to make his point, and thus implicitly claimed that moral qualities were analogous to or were a sort of qualia. Moore argued that scientists can attempt to define yellow with a naturalistic description—such as "light with approximately 600 nm wavelength" (McMurry and Fay 1995, 147)—but any such definition would not capture what yellow is. In other words, no definition using natural concepts could capture the essential properties of yellowness. In Moore’s words:
Consider yellow, for example. We may try to define it, by describing its physical equivalent; we may state what kind of light-vibrations must stimulate the normal eye, in order that we may perceive it. But a moment’s reflection is sufficient to shew that those light-vibrations are not themselves what we mean by yellow. They are not what we perceive. Indeed, we should never have been able to discover their existence, unless we had first been struck by the patent difference of quality between the different colours. The most we can be entitled to say of those vibrations is that they are what corresponds in space to the yellow which we actually perceive.
The same goes for moral concepts, according to Moore. Defining the good as what produces the greatest amount of pleasure for the greatest number of sentient beings in fact fails to capture what is good, although it might track it.
Yet a mistake of this simple kind has commonly been made about good. It may be true that all things which are good are also something else, just as it is true that all things which are yellow produce a certain kind of vibration in the light. And it is a fact, that Ethics aims at discovering what are those other properties belonging to all things which are good. But far too many philosophers have thought that when they named those other properties they were actually defining good; that these properties, in fact, were simply not other, but absolutely and entirely the same with goodness. This view I propose to call the naturalistic fallacy and of it I shall now endeavour to dispose.
Thus Hume’s is-ought fallacy was reborn with Moore’s introduction of the naturalistic fallacy. But how exactly did Moore prove that the naturalistic fallacy was a fallacy?
Moore convinced his contemporaries that the naturalistic fallacy was a fallacy by an argument called "The Open-Question Argument" presented in Principia Ethica. According to Moore, a term is defined just in case we provide necessary and sufficient conditions for using the term. Moore’s was the traditional view of definition; it was developed by Aristotle (384-322 B.C.E.) as a way to describe the natural essences that things had. As it happens, terms that are defined in this traditional way form closed questions with their definitions.
For example, since "bachelor" is defined as "unmarried man," the following is a closed and thus nonsensical question: I know he is a bachelor but is he unmarried and a man? Moore’s insight was seeing that terms that were not defined by a certain phrase formed open questions, and naturalistic definitions of moral concepts fell within this category.
For example, Moore saw the following as a completely open question: I know she is doing what produces the greatest pleasure for the greatest number of sentient beings, but is she doing something good? Particularly, endorsing American slavery in the eighteenth century would have produced the greatest amount of pleasure for the greatest number of American people (since American whites outnumbered American blacks in the eighteenth century); however, it seems far from obvious that endorsing any sort of slavery could ever be good.
For this reason Moore rejected moral naturalism and proposed "moral intuitionism" in its place. This philosophical doctrine claims that moral terms are indefinable and rather we understand moral concepts through moral intuition.
Despite its intuitive appeal, several philosophers have directly or indirectly attacked the naturalistic fallacy. To be sure, the naturalistic fallacy rests on two major and controversial assumptions. First, it assumes that moral concepts must be sharply defined with necessary and sufficient conditions. Second, it assumes that a concept’s meaning lies in its description instead of its reference. However, philosophers have questioned both of these assumptions in the philosophy of language over the course of the twentieth century.
Richard Boyd (1988) questions whether moral terms must have sharp definitions in order to be defined. In the end he discovers that moral terms can have what he calls “homeostatic cluster definitions,” which are vaguely defined terms using criteria that are neither necessary or sufficient. In fact, Boyd (1988) argues that cluster definitions are commonplace in human languages. He uses biological species terms (e.g., "Homo sapiens") as an example.
Thus the fact that a moral term and its naturalistic definition do not form a closed question does not imply that defining moral concepts with natural concepts is a fallacy, since moral concepts could be vaguely and imprecisely defined homeostatic cluster concepts. In Boyd’s words:
Similarly, consider the objection that a moral realist must hold that goodness is a natural property, and thus commit the “naturalistic fallacy” of maintaining that moral terms possess analytic definitions in, say, physical terms. The moral realist may choose to agree that goodness is probably a physical property but deny that it has any analytic definition whatsoever (Boyd 1988, 199).
The first philosopher to question whether definitions needed to be sharp was Ludwig Wittgenstein (1889-1951) who in 1951 proposed family resemblance concepts as a vague alternative to sharply defined concepts in his classic text Philosophical Investigations. Although Wittgenstein suggested that moral concepts were family resemblance concepts, he did not develop a detailed philosophical theory on the issue like Boyd.
Even more important in twentieth-century philosophy of language was the proposal of an alternative theory on the meaning of meaning (or the definition of definition). Since Aristotle, philosophers have thought that the meaning of a word lies in a description of it using other words. For example, a bachelor is an unmarried man. However, Saul Kripke (1972) and Hilary Putnam (1973) offered a different view on the meaning of meaning. They claimed that the meaning of a term (e.g., a name) could be its referent instead of its description. The favorite example for philosophers in this camp is "water is H2O."
We need not delve into the reasons for why some philosophers believe in the referential theory of meaning in order to present it as an alternative way to understand meaning. Thus naturalistic philosophers can challenge the naturalistic fallacy in a second way; they can reject the naturalistic fallacy on the grounds that moral terms can be defined referentially instead of descriptively (Boyd 1988).
Despite these sophisticated challenges for the naturalistic fallacy achieving the status of a legitimate fallacy in moral reasoning, we can still point out how it highlights fallacious reasoning in some form.
First, someone who believes in the descriptivist theory of meaning and that moral concepts have sharp definitions certainly commits the naturalistic fallacy if she commits the is-ought fallacy. Thus the naturalistic fallacy appears to be a legitimate fallacy for ordinary people who engage in certain ordinary moral reasoning. The big question is whether experts in moral philosophy (e.g., meta-ethicists) are able to commit the naturalistic fallacy if their theories on language are sophisticated enough. However, the answer to this question is uncertain and is an active area of research in contemporary metaethics.
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