Our conceptions of how the organs of the body work are primarily derived from experiments done on muscle, the organ from which experimenters have been accustomed over many decades to ascertain the fundamental properties of living tissues; the principles there learnt have then been directly applied to the problems presented by other organs. Such having been, and still being, scientific practice, it follows that, if we find out about the working of muscle something fundamentally different from that hitherto suspected, we not only obtain therefrom new ideas of the working of muscle, but also new principles to apply to our ideas of the working of other organs. It could happen, however, that new knowledge concerning the fundamental working of the organs of the body should actually come from some other organ than muscle. In that case the newly discovered phenomena would not be directly explicable in terms of the fundamental principles derived from muscle. Two courses would then be possible. The discoverer could re-consider his fundamental principles, and thereby be led to reexamine the workings of muscle in the light of the information supplied by the other organ, or he could frame an ad hoc hypothesis concerning the supposed peculiar behaviour of the other organ. The latter has been the usual course followed, though it would not appear that the framing of such hypotheses has been made with full awareness that they really resolve conflict between principles derived from muscle and principles derived from the other organ.