One of the things I attempted to do in my paper on independent testability (1980) was to illustrate convincingly the very real difficulties of distinguishing between the accidental and essential features of a scientific experiment. The importance of this distinction is that independent testability presumably requires differences that are essential and telling and not merely accidental reflections of existing experimental technique or of the procedural preferences of the experimenter. In the case of the Michelson-Morley (MM) and Kennedy-Thorndike (KT) experiments, I showed, following a suggestion of M. G. Evans, that Grünbaum's attempt to distinguish these two experiments does not work. I also showed that certain obvious and natural variations of Grünbaum's approach do not adequately distinguish these experiments. So, for example, it will not do simply to claim that MM is characterized by equal arms and KT by unequal arms, since equal arms are required for MM only in the absence of adequate photographic registration methods. But if these methods are not available then KT is not possible. Conversely, if these methods are available, then while both MM and KT are experimentally possible, MM need not be restricted to equal arms.

Frankel (1976) has argued that the theory of causality developed by Rom Harré (Harré 1970) and his colleague Edward Madden (Harré and Madden 1975) is incoherent, since the proposal that causal claims are naturally necessary leads to a vicious infinite regression

“which ends by requiring that for any causal claim to be accorded the status of natural necessity an infinite number of causal claims must be accorded the status of natural necessities.” (Frankel 1976, p. 560)