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. The argument clearly depends on the assumption that if 'good' were definable, it would be an analytic truth about 'good', an assumption that many contemporary moral realists like Richard Boyd and Peter Railton reject. 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).