Running Head: What Is Disease?
Article type: Scientific Contribution
For: Medicine, Health Care and Philosophy
On the Difficulty of Defining Disease:
A Darwinian Perspective
Randolph Nesse, M.D.
The University of Michigan
Address correspondence to:
Randolph Nesse, M.D.
Professor of Psychiatry,
The University of Michigan
Institute for Social Research, Room 5057
426 Thompson Street
Ann Arbor, MI 48106-1248
(734) 764-6593 Fax: (734)647-7340
Most attempts to craft a definition of disease seem to have tackled two tasks simultaneously: 1) trying to create a series of inclusion and exclusion criteria that correspond to medical usage of the word disease and 2) using this definition to understand the essence of what disease is. The first task has been somewhat accomplished, but cannot reach closure because the concept of “disease” is based on a prototype, not a logical category. The second task cannot be accomplished by deduction, but only by understanding how the body works and what each component is for, in evolutionary detail. An evolutionary view of the origins of the body and the vulnerabilities that result in disease provides an objective foundation for recognizing pathology. Our social definition of disease will remain contentious, however, because values vary, and because the label “disease” changes judgments about the moral status of people with various conditions, and their rights to medical and social resources.
The problem of defining disease has occupied so many good minds for so long with so much continuing contention, that it seems foolhardy for a nonspecialist to even venture thoughts on the issue. However, the very lack of consensus suggests that the question may be either miscast or unanswerable. Given this situation, it seems worthwhile to examine the question from the perspective of a secure scientific foundation, that of evolutionary biology. (Nesse and Williams, 1995)
As best I can understand it, the quest has generally been for a series of words that define disease in a way that matches conventional use of the word and that provides insight into the essence of disease. (Humber and Almeder, 1997) These two aims are actually quite distinct, and may require very different approaches. The first aim attempts to define disease by crafting phrases whose intersections create a Venn space that encircles all that we mean by disease and nothing else. The enterprise has made considerable progress, with most authorities specifying a state of the body that differs in disadvantageous ways from species typical states. All of these definitions, however, fail in some respect or another to fully satisfy the criterion of matching common medical usage. For instance, some include pregnancy as a disease, but exclude upper respiratory infections. (Clouser, et al., 1997) I suspect this difficulty arises because human concepts like disease refer, not to categories defined by logical inclusion and exclusion criteria, but to prototypes. In the case of disease, this prototype approach makes great sense. The disease concept must have emerged when people tried to communicate to each other that something was wrong with their bodies, whether from pneumonia, cancer, infected wound, or kidney stones. People with no idea about microbes, genes, or even anatomy, must have used the concept of disease to refer quite generally to any undesirable bodily condition, and perhaps mental conditions as well. In those days, such conditions could be identified only by suffering, disability, or cues that indicated increased vulnerability to suffering or disability. From this vantage point, the very origins of the disease concept involve a value judgment—suffering and disability are undesirable. Whether this value judgment exists only in our evolved minds is another matter, but our negative judgments of suffering and disability are by no means arbitrary, having been shaped by natural selection. The utility of pain as a motivator of escape and avoidance depends, after all, on its intrinsic aversive character.
The second goal has been to use a definition of disease to gain insight into its nature, often with the implicit assumption that disease exists as an ideal form. In recent writing, this question seems to have been posed as a choice between naturalistic views of disease, such as those of Boorse, versus normative positions, as proposed by Englehardt. (Boorse, 1997; Engelhardt, 1996, pp. 189-238) I can see the issue, but it is not clear to me that these positions are mutually exclusive alternatives. (Kovács, 1998) Disease can refer to specific objective bodily conditions, and yet the interpretation of those conditions, and their valuation, may vary from culture to culture. Furthermore, the social significance of defining a condition as a disease varies considerably. The separate, and to my mind more fundamental question, is whether there is some objective way of specifying when a body is abnormal and when it is not. This is probably not susceptible to deductive analysis, but requires, instead, a full knowledge of how every aspect of the body works, how it contributes a fitness advantage, and how it was shaped by natural selection. In most instances, our knowledge about the body provides a basis for strong opinion, in many cases a definitive opinion. Nonetheless, we don’t understand everything, so we cannot always determine whether a condition is a disease or not. If we did have a full and detailed understanding of every aspect of the body and its origins, this would, I believe, provide an objective basis for determining if any given condition is pathological or not.
Interestingly, debates about the definition of disease often mix these two goals. Actually, three goals could be identified, if one wanted to consider the social significance of the label “disease,” separately from the mere designation of a condition as a disease. For instance, Boorse claims that disease can be defined apart from social values, but his definition relies on statistical normality rather than to any biological criterion, and he assesses the quality of a definition by how well it conforms to how doctors use the word disease.
We have already noted that the concept of disease undoubtedly arose early in human speech and cognition to describe undesirable bodily conditions. People want their bodies to be healthy instead of sick, and they created words to describe these desirable and undesirable states. As time went on, however, several other questions arose. Even if we know what we mean by disease, what is it? Where does it come from? Why does it exist at all? What can we do about it? Is it necessary? Can it be prevented? Now instead of mere definition, we need a causal analysis. The core simple notion of disease indicates that something is wrong with the body. Some mechanism is not working correctly. This implies, of course, that we know what “working correctly” means. It suggests that we know how to recognize normality. Often, of course, we can. Cancer is abnormal, shivering is normal. But as soon as we inquire deeply, our intuition fails us. Is depression an adaptation or a disease, given that natural selection may have shaped system to turn off motivation in situations where action is likely to be maladaptive? (Nesse, 2000) Diabetes of pregnancy is a disease for the mother but it seems to exist to benefit the fetus. (Haig, 1993)
The difficulty here is understanding what we mean by normal. Reference to statistical norms is helpful, especially if care is taken to adjust these norms for age and sex. (Boorse, 1997) Nonetheless, tooth caries and heart disease are statistically normal in many cultures, but still abnormal because they interfere with function. Other conditions, such as manic-depressive illness, may even give a selective advantage, and yet be abnormal. A deep understanding of what is abnormal requires a richly detailed understanding of what is normal. Statistical generalizations will not suffice. Instead, we require nothing less than a complete knowledge of what the body is for, how it works, and, especially, how it came to have its current form. If we had this knowledge in hand, then we could define abnormality with reference to deviations from normality, not needing to resort to either statistical or value laden information. In some cases pathognomonic signs define the presence of a disease. (Wulff, 1981) The problem is, of course, that not all diseases can be sharply distinguished from normal. Our knowledge is limited. On the other hand, we now know a great deal about he design and function of the body and recently, we have gained a much better understanding of how the body was shaped by natural selection and for what. The rest of this article will attempt to determine how helpful this knowledge can be in deepening our understanding of what disease is.
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