Popper's explications of ad hocness: Circularity, empirical content, and scientific practice

Karl Popper defines an ad hoc hypothesis as one that is introduced to immunize a theory from some (or all) refutation but which cannot be tested independently. He has also attempted to explicate ad hocness in terms of certain other allegedly undesirable properties of hypotheses or of the explanations they would provide, but his account is confused and mistaken. The first such property is circularity, which is undesirable; the second such property is reduction in empirical content, which need not be. In the former case, I argue that non-circularity is in any event preferable to non-ad hocness as a necessary condition for a satisfactory explanation or an explanans, as the case may be, and I try to sort out various persistent errors surrounding this comparison. In the latter case, I suggest that Popper is barking up the wrong tree, that important scientific progress sometimes does consist in just such reductions in empirical content as he proscribes. This provides a further reason for not taking ad hoc hypotheses as Popper conceives them to pose the danger for science he believes they do