What is the nature of the logical relationship between health, illness, and disease? Many health-care professionals and a large percentage of the general public appear to view health and disease as logical opposites, while illness and disease are often used as synonyms. When asked if they are healthy or feel well, most people will answer affirmatively if they are not suffering from any particular disease or disorder at the time the question is asked. Looked at uncritically, health would seem to be no more than the absence of disease and illness. Disease and illness appear to connote any impairment in a person's sense of well-being or fitness.

But there are powerful reasons for questioning the appropriateness of viewing health and disease merely as conceptual opposites, and disease and illness as synonyms. Even if no particular disease is present, it is still possible to say that a person seems healthier or is healthier in certain ways than others. The average marathon runner or professional athlete is probably healthier, with respect to overall physical well-being, than the average philosophy professor, even if neither happens to be afflicted with a specific disease at the time the comparison is made. Health does seem to require the absence of disease or illness as a necessary condition, but it is not clear that this absence is by itself sufficient to define the nature of health.

The possibility that health is not simply the absence of disease or illness, but refers to something more, is reinforced by the activities of some contemporary health-care practitioners. Some see their job as more than simply the alleviation of disease. Psychoanalysts, plastic surgeons, nutritionists, and sports physiologists are interested not only in the prevention or alleviation of disease, but also, and perhaps sometimes only, the promotion of health. If health and disease were logically related to one another as contradictory concepts, it would be impossible to make any sense of the ideas expressed by such terms as 'maximize health' or 'positive mental health'. These ideas, as well as the notion that relative degrees of health can exist even in the absence of disease or illness, do appear to be meaningful and coherent.

Not only is the absence of disease not sufficient to establish the existence of health, diseases do not always impair or threaten health. Some diseases are unpleasant and disabling but do not compromise the health of the individual who has them. For example, enduring a short bout of measles or mumps during childhood, either through infection or inoculation, may actually be conducive to health. A person can be riddled with cancer, delusional, or suffer from hypertension, and yet remain entirely unaware of any symptoms of dysfunction. A person who is the carrier of the sickle-cell trait, while prone to certain problems under certain rare circumstances, is better protected against malaria than someone lacking this genetic endowment. The fact that one can be functioning well and feel healthy while suffering from a disease also hints that the concepts of illness and disease may actually refer to different states or conditions. Feeling sick is not the same as having a disease, since one can be ill but not diseased and diseased but not feel ill.

Some behaviours that are viewed as diseases in at least some circles - gambling, homosexuality, hyperactivity, or drug addiction - have tenuous logical connections to the concept of health. Certainly, it is much less difficult to obtain agreement across social classes and different cultures about those states of the mind and body that constitute diseases than it is to secure agreement about which states are to be viewed as healthy. The use of addictive narcotic drugs and their relationship to health is viewed quite differently in Jamaica, Holland, Bolivia, and Peru than it is in the United States or Saudi Arabia.

While the view of health and disease as conceptual opposites may be widely accepted, there are reasons for calling this presumption into question. A strong case can be made for the view that the logical relationship between health and disease is not one of contradiction. The conceptual opposite of health might more reasonably be seen as unhealthy; and the logical contradictory of diseased as non-diseased. Health and disease may exist as parallel concepts rather than as concepts defined only in terms of one another. While disease may be among the criteria that are used to define health, other measures or states may be necessary in order to attribute this state to an individual.