Definist fallacy

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The definist fallacy involves the confusion between two notions by defining one in terms of the other. G. E. Moore, for example, argued that "good" should not be defined in terms of "producing pleasure", because it would always make sense to ask of a pleasureable thing whether or not it was in fact good (see the Open Question Argument).

It is in fact unclear that the definist fallacy is fallacious, as the fact that there is always an open question merely reflects the fact that it makes sense to ask whether two things that may be identical in fact are. Thus, even if the good is identical to what is pleasurable, it makes sense to ask whether it is; the answer will be "yes", but the question was legitimate. That is, Moore can't deny that, for example, it makes sense to ask whether Hesperus is the same heavenly body as Phosphorus, even though we know that it is. Moore seems to be implying that it might not be, that the answer to "are they the same" might be "no" — but that is to beg the question against their identification.

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