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The Open Question Argument is a philosophical argument put forward by the British philosopher G. E. Moore in his Principia Ethica. It sets out to demonstrate the indefinability of the predicate "good" and thus show up what he called the "Naturalistic fallacy". 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. Others hold that it may be reasonable to assert, however, that the term "good" is merely an affirmation of approval, and that, as such, good my be defined as "I approve". In this context, questions such as "I see that this is pleasant, but is it good?" translate as "I see this is pleasant, I approve of pleasure". This however may raise the question "Why do I approve of pleasure?" this may lead to analysis of what pleasure means to living_organisms to consider whether there is a purpose in seeking pleasure, which may lead to analysis of value in terms that relate to living organisms.