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.
Moore advanced an argument for the indefinability of “good†(and demonstrating the “naturalistic fallacyâ€) which is known as the Open Question Argument.
