What we are confronted with in nineteenth-century ether theories, then, is a wide
variety of once successful theories, whose central explanatory concept Putnam singles
out as a prime example of a nonreferring one.' What are (referential) realists to make
of this historical case? On the face of it, this case poses two rather different kinds of
challenges to realism: first, it suggests that (S3) is a dubious piece of advice in
that there can be (and have been) highly successful theories some central terms of which are
nonreferring; and second, it suggests that the realist's claim that he can explain why science
is successful is false at least insofar as a part of the historical success of science has been success
exhibited by theories whose central terms did not refer.