The difference between a medical cause in the generalised sense, (where cause means a defining correlative for a concept), and a medical cause in the particular sense (when we seek to find out or state the antecedent without which this man would not be as he is here and now) is one of enormous importance, and one that should be constantly borne in mind when discussion is commenced. It is true that it involves the oldest of logical and metaphysical problems in respect of scientific thought――the question of universals and particulars; but that does not make it any the more easily shirked. Its relevance to the question of cancer is this: that the proof of the production of cancer in men or in animals _under one set of circumstances_ does not warrant us in saying that that set of circumstances _as known to us_ involves _all_ the factors without which cancer cannot occur. And, even if research work demonstrated that, in every case now called cancer, some parasite or growth-form, some irritating factor that can be isolated, does actually obtain, _unless it could be shewn that this parasite or factor is never found except where there is cancer as we now define it_, we should have to proceed to investigate why and how cancer does not always occur when this factor is present. Just so are we at present seeking to explain why and how, of so many persons exposed to infection by Koch’s bacillus, only certain ones do become diseased. If we find that only those persons who possess a character that we may call “X” become infected, we shall then have to say that, not Koch’s bacillus, but the character “X” is “the” cause of tuberculosis. It is thus that science progresses: not by making the absolute and positive discoveries that the public is taught to expect, but by arranging and rearranging our experiential knowledge, as such grows, in terms of so-called laws and generalisations, that are found _progressively convenient_. But such laws and generalisations are not necessarily the one more “true” than the other, _except in relation to the knowledge that they summarize_. If such considerations as these were more frequently borne in mind, there would be less unconscious deception, less disappointment, and greater economy in work and thought.