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CHAPTER 1 THE LOGIC OF THE COMPARATIVE METHOD Ordinarily we do not reason deliberately following the rules of logic, and we are not consciously aware of what makes our thoughts flow this way or that. I believe that the importance of the part played by analogy in our thinking— scientifically and otherwise—is not sufficiently appreciated. Very commonly our actions are the consequence of our regarding a new situation as analogous to one with which we are familiar. An analogy is a resemblance between the relationships of things, rather than between the things themselves. When one perceives that the relationship between A and B resembles the relationship between X and Y on one point, and one knows that A is related to B in various other ways, this suggests that one should look for similar relationships between X and Y. Clearly there is no certainty about this sort of reasoning . The value of analogies is that they are suggestive, not that they prove anything, and often they provide the only form of reasoning available. In any case it is as well to remember that in research much of our thinking inevitably is specu21 Frontiers in Comparative Medicine lative. It is a fallacy to suppose that we can prove a theory in any absolute sense. One can only prove that something happened in the past; a theory must be concerned with past, present , and future if it is to be of any value. The usual procedure in research is to accumulate evidence which more and more people, judging it subjectively, find convincing, until eventually the theory is accepted by practically everyone. Strictly speaking, comparative medicine means comparing disease phenomena in different species, including man, but in the present discussion I regard it as meaning research by analogy between disease in man, on the one hand, and disease in the rest of the animal kingdom, on the other. When we see that an animal disease and a human disease have certain characteristics in common, we are led to think they may have also other characteristics in common. Having recognized that the clinical and pathological manifestations of the two diseases are similar, we reason by analogy that if we can find out more about the animal disease, the knowledge so gained can be applied to the human problem. This is the logic behind using animal models in medical research. However, this bare outline may lead to an oversimplified view which can give rise to misunderstandings. Comparative medicine usually involves more than just taking results obtained in research with animals and applying them directly to the human problem, that is to say, extrapolating them. Research on animals enables general principles to be discerned, and it is these that are transferred to the study of the human problem. By "general principles" I mean theoretical concepts, understanding of the physicochemical mechanisms involved, and the development of technical methods. To put it another way, comparative medicine indicates what we should look for in man and how to go about it. In practice, the comparative method may be more complex 22 The Logic of the Comparative Method than it appears in retrospect when the problem can be seen clearly. Sometimes the problem is to find meaningful analogues , that is, models. There may be no difficulty with clearcut situations, such as an infectious disease or nutritional deficiency disease, if it occurs naturally in some other species as well as in man. With some human diseases, however, it isdifficult to find an analogous condition in an animal, or the difficulty may be to decide whether a disease in an animal is sufficiently similar to be a useful model. In such situations there is much scope for personal judgments and consequently for differences of opinion. I do not wish to suggest that the advances in comparative medicine are always due to research planned in advance. Many discoveries have been made in the course of investigations carried out on diseases of domestic animals because of the economic importance of those diseases, without any intention of throwing light on human diseases, and only subsequently was it found that the new knowledge could be transferred to analogous human diseases. In comparative medicine, as in most fields of research, new knowledge is won in either of two ways: by a planned effort to solve a recognized problem or by opportunism, that is, seizing on an unexpected finding and following up its implications. Origin of New Concepts of Communicable Disease I shall give some examples of...


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