Of course, the referentialist response shows only that reference can be retained, not that it must be.  Consequently it is only a partial defence of realism against semantic incommensurability.  A further component of the defence of realism against incommensurability must be an epistemic one.  For referentialism shows that a term can retain reference and hence that the relevant theories may be such that the later constitutes a better approximation to the truth than the earlier.  Nonetheless it may not be possible for philosophers or others to know that there has been such progress. Methodological incommensurability in particular seems to threaten the possibility of this knowledge.  Kuhn thinks that in order to be in a position to compare theories from older and more recent periods of normal science one needs a perspective external to each and indeed any era of science–what he calls an ‘Archimedean platform’ (1992, 14).  However, we never are able to escape from our current perspective. A realist response to this kind of incommensurability may appeal to externalist or naturalized epistemology.  These (related) approaches reject the idea that for a method to yield knowledge it must be independent of any particular theory, perspective, or historical/cognitive circumstance.  So long as the method has an appropriate kind of reliability it can generate knowledge.  Contrary to the internalist view characteristic of the positivists (and, it appears, shared by Kuhn) the reliability of a method does not need to be one that must be evaluable independently of any particular scientific perspective. It is not the case, for example, that the reliability of a method used in science must be justifiable by a priori means.  Thus the methods developed in one era may indeed generate knowledge, including knowledge that some previous era got certain matters wrong, or right but only to a certain degree. A naturalized epistemology may add that science itself is in the business of investigating and developing methods.  As science develops we would expect its methods to change and develop also.