Exploring the Biological Meaning of Disease and Health

Abstract

Although biology has come to be seen as the scientific basis for much of modern medicine, this association is historically very recent and is, as yet, incomplete. Evolutionary theory, the fundamental explanatory paradigm upon which modern biology now rests, is even more recent and continues to be explored and to arouse vigorous professional debate. Only now is evolutionary theory beginning to enlighten medical understanding and the ethical implications of this have yet to be investigated. Key to this is our understanding of disease and health but finding definitions for them has proved impossible.

With reference to ideas from a range of thinkers, ways of comprehending the human biological experience of disease and health will be reviewed. It will be suggested that to at least understand disease and health better, there is a need to question our presuppositions about ourselves.

Likewise, problems that might be caused by the application of evolutionary theory to medicine might also benefit from this approach. Although the evolutionary paradigm can be applied to the human species, the question will be raised whether the same version of that paradigm is indeed fully applicable or whether a biology of the individual needs consideration.

There is a story about the physicist Enrico Fermi (1901-1954). Thanking a guest speaker for a lecture he had just given, Fermi told him that before the lecture he had been confused about the subject. Now, having heard him, he was still confused - but at a higher level.

Confusion - or at least, a lack of certainty - is, I believe, a very important thing. Without such as these, there is no cause for investigation. Therefore, I hope to cast some doubts in your minds about things with which we are very familiar; things about which we are usually quite certain.

In connection with my current work, in the mid-1990s, I experienced two significant things. The first was the publication by Nesse and Williams of the book they entitled 'Why we get Sick' - which was published in the UK under the title 'Evolution and Healing'. This book plugged a gap for me and literally sent me in a new direction. I had been aware, since I was an undergraduate, that disease was not receiving sufficient attention from an evolutionary perspective and had wondered about ways of linking them. Now I could see a way ahead. With that book, and Randy's subsequent, almost crusading, zeal, this is now being given serious consideration. Now the new field of 'Darwinian (or 'Evolutionary') Medicine' is itself evolving. I hesitate to use the word 'established' because these are still early days; there is still much to be done. Not least, in understanding the conceptual aspects of 'Evolutionary Medicine' - this is essentially my theme.

The other event that I experienced in the mid-1990s was being asked to teach a postgraduate course entitled 'The Biology of Degenerative Disease'. This caused me a significant problem: What exactly was 'Degenerative Disease'? I was taking over this course from somebody else; I could very easily have copied what had been taught previously. But I did not find that a particularly stimulating prospect. From my searches, I could find only one textbook that used the term in its title - and that was a veterinary text. The more I thought about the title for myself, the more difficult it became to say exactly what it really meant. For example, cancer is a disease but is it degenerative? Aging is associated with degeneration but is 'aging' a disease? These were the sort of questions I asked myself.

This may sound a little strange. Degeneration seems straightforward enough and after all, we all know what 'disease' is – until we come to define it, that is. Then we have problems. Every year, on one of my courses, I give my students the task of doing just that. I ask them to go off and work in groups and then to come back with a definition of 'disease' and a definition of 'health', amongst other things. It is an interesting exercise. The more academically able the students, the less likely they are to be able to do the task.

In his 'Confessions', St Augustine seems to have had a similar problem with the concept of time. He knew what time was, until he came to say what it was precisely - then he could not. I soon found that I was not alone in my problem. Consider this quote:

'In the last quarter of the 20th Century we have been witnessing an intense discussion on the nature of health ... and disease and on the meaning of these ... basic notions of medicine.' (Sadegh-Zadeh, 2000)

This discussion has become an extensive and complex debate. Some, having tried to define 'health' and finding it impossible, have turned their attention to disease, arguing that if health could be defined as the absence of disease then defining disease allows an alternative way of getting to a definition of health. But defining 'disease', they have found, is problematic.

Looking at my title, I have put 'Disease' before 'Health' when an English speaker would normally put them the other way round. I have done this to make the words more conspicuous. Secondly, I have used the word 'meaning' - which is not a common word in science - but I have prefaced it with 'biological' lest anybody think that I am inferring some form of metaphysical meaning. But, at the same time, I hope this leaves some unease about the phrase. The words 'biological' and 'meaning' do not tend to go together. And finally, I use the word 'exploring' because, when it comes to 'the biological meaning of disease and health', I find that the best I can do is to probe for possibilities rather than make definitive or authoritative pronouncements. As I implied earlier, I am here to be thought-provoking rather than instructive.

The concepts of 'disease' and 'health' are held to be central to modern healthcare having a bearing on individuals and society. A variety of social and economic rights, entitlements, exemptions and accountabilities are affected by whether a person is deemed healthy or otherwise. A way of ascertaining this would therefore seem to be essential. The more so if we consider that homosexuality, masturbation, black (or Negro) skin and the tendency of American slaves to run away from their masters, have, at one time or another, all been labelled 'disease' and have elicited various treatments - some quite unpleasant and applied against the will of those supposedly afflicted. The concepts of 'disease' and 'health' are also held to be important in the identification and treatment of disease states and to help to clarify the goals and limits of individual therapies and medicine as a whole.

As noted in the program booklet, 'Evolutionary Medicine is "the enterprise of trying to find evolutionary explanations for vulnerabilities to disease" (Nesse)'. Thus, Evolutionary Medicine would seem to need a concept of 'disease' or else it would be unclear what it was that we are 'trying to find evolutionary explanations for vulnerabilities to'.

Although there are many nuances of opinion, it is probably true to say that there are two over-arching schools of thought when it comes to the concepts of 'disease' and 'health'. These are referred to as the 'naturalist' and the 'normativist'. The naturalists argue that health is the absence of disease and that the term 'disease' can be understood objectively as biological functioning that is statistically below normal for the species concerned. Accordingly, they believe, this definition is uninfluenced by external human values. Naturalism tends to attract scientists or at least to be most closely associated with their way of thinking. 'Normativists' give emphasis to the subjective nature of disease experience, which differs between cultures and through history. The classification of certain biological phenomena as diseases, it is argued, is based upon value judgements. The concept of disease is normative - where, what counts as the 'norm' is prescribed rather than statistically derived. In effect, we decide what constitutes a disease.

At the heart of the debate - although easily overlooked - are the questions 'What do we mean when we use the word "disease"?' and 'What do we mean when we use the word "health"?' Sometimes the debate seems to be merely about our use of those words. Sometimes little consideration is given to the underlying biology. This is an omission that needs serious attention.

I should say that my own interest in disease is largely confined to the physical or organic. However, as I hope will become evident later on, that interest is one that pays particular attention to its conceptual aspects. The quote I used earlier was incomplete.

'In the last quarter of the 20th Century we have been witnessing an intense discussion on the nature of health ... and disease and on the meaning of these ... basic notions of medicine. The discussion seems to have ended up a blind alley, however.' (Sadegh-Zadeh, 2000)

We all know what 'disease' and 'health' are - or so we think; we all know what we mean when we use those terms, or so we think; but pinning down exactly what we mean is another matter. Outside of the technical debate there is also the general attitude that 'disease' relates to some form of abnormality - and conversely health is associated with normality. This has been called 'naïve normalism' and is an attitude to be found implicit in many textbooks aimed at doctors, nurses and other health professionals. Indeed, it has also been suggested that this is the prevailing attitude in most schools of medicine, nursing etc. Here 'disease' and 'health' are usually seen as the opposite poles of a continuum.

Also there is a tendency to think that pathologically altered tissue is equivalent to 'disease'. This is not so. This forgets that, in medicine, there is the notion of the 'pathological counterpart' to a disease. A mass of excised pathological tissue may have been the source of an individual's 'dis-ease' but it was not the disease per se. It is easy to slip into that way of thinking and talking. For a given individual, a disease is broader than altered tissue. We rightly talk about 'evolutionary explanations for vulnerabilities to disease' not 'vulnerabilities to pathology'. Many of the body's reactions associated with disease - and which also make us feel uncomfortable - ultimately do us good. In coronary artery disease, for example, the lumen of a coronary artery can be reduced by something like 70% before an individual becomes aware of any symptoms. The association between biological or physical change and an individual's experience of disease is not simple and can vary markedly from one individual to another.

Instead of a single axis, what if we use two axes: one for 'experience', the other for 'pathology'? Under 'pathology' here we can include the activity of pathogenic organisms for example. Here we have four quadrants to fill.

That associated with a lack of pathology and a sense or experience of well-being seems straightforward enough, as does that associated with the presence of pathology and a lack of a sense of well-being. These two states are typically those assumed by a 'naïve normalist' description. We have a 'normal' state to the lower right and an 'abnormal' state to the upper left. But we can also have the presence of pathology while still having a sense of well-being - as in the case of the gradually occluding coronary artery just mentioned. Likewise, there can be an absence of pathology and a lack of a sense of well-being. This quadrant accommodates those mental diseases for which there is no organic basis.

As I said earlier, my own interest in disease is largely confined to the physical or organic. But having mentioned mental disease (more commonly referred to as 'mental illness'), let me briefly note that many have attempted to define 'disease' and 'health' without separating the way these terms are used to describe organic conditions from the way they are used to describe mental conditions; they have attempted to find definitions of these terms that can be applied equally to both organic and mental states. This, I believe, clouds the debate somewhat. Notwithstanding the mind-body problem, organic and mental states may be essentially different and not warrant the use of the same words - or, at least, the use of the same words in the same way. It should also sound a note of caution for Evolutionary Medicine. It may be wiser to treat 'evolutionary explanations for vulnerabilities to organic disease' quite separately from 'evolutionary explanations for vulnerabilities to mental disease'.

To return to this bi-axial graph of pathological presence and sense of well-being - by exploring in this way, we are beginning to do something that could help get us out of the blind alley mentioned earlier. In effect, we need to find a new or different way of looking at our problem. Separating 'experience' and 'pathology' begins to do that. (Although whether it actually leads to a solution is another matter.) In effect, what I am describing here is what the Nobel laureate Sir Peter Medawar called 'Kantian Experimentation'.

In his Jayne Lectures of 1968, which have been published as 'Induction and Intuition in Scientific Thought' (and which also appear in his book 'Pluto's Republic'), he described four experimental types, which he named after famous thinkers. There is the Baconian (or inductive) type of experiment (after Sir Francis Bacon (1561-1626)) characterized by the phrase "I wonder what would happen if ...?" All investigations, Medawar suggests, begin this way. There is the Galilean (or critical) type of experiment (after Galileo Galilei (1564-1642)) where (in Medawar's words) "Actions are carried out to test a hypothesis or preconceived opinion by examining the logical consequences of holding it." This was the type of experiment he considered to be central to science and the means of applying Popper's hypothetico-deductive system - of which he was a champion. There is the Aristotelian (or demonstrative) type of experiment (after Aristotle (384-322 BC)) "Intended to illustrate a preconceived truth and convince people of its validity."

Then, there is the Kantian (or deductive) type of experiment (after Immanuel Kant (1724-1804)). This is based on the idea: "Let's see what happens if we take a different view". This type consists of "

Experiments in which we examine the consequences of varying [our] axioms or presuppositions". They might be described as 'thought experiments' but not as '"mere" or "simple" thought experiments' - there is nothing about them that makes them easier than laboratory experiments. In fact, they can be a great deal harder.

In his recent talk for the KLI, Randolph Nesse may have suggested something like the following syllogism:

· Biology is the basic science for Medicine

· Evolution is fundamental to understanding Biology

Therefore

· Evolutionary Biology is a basic science for Medicine

If so, the exact wording will have been different. I have adapted this from some notes he kindly gave me following a conference we attended in London last December. I do not disagree with this syllogism but, upon interrogation, it yields certain insights that, I believe, Evolutionary Medicine will do well to take on board.

Leaving aside the second premise, that 'Evolution is fundamental to understanding Biology', I want to focus on the first, that 'Biology is the basic science for Medicine'. For it is here that we need to question certain axioms and presuppositions about the relationship between biology and medicine.

Interestingly, all cultures have some form of medical practice. The practice of medicine is as old, if not older than modern humans. Consider our cousins the Neandertals. Life for them was clearly harsh. Four out of six adult Neandertal skeletons found in a cave near Shanidar, Northern Iraq, show signs of pathology or injury. The specimen called Shanidar 1 showed a severely atrophied (or withered) right arm, a condition that he had carried for most of his life, possibly since birth. He also had a crippled and withered right leg. One of the metatarsals on his right foot shows a healed fracture. He had also suffered a crushing injury that probably blinded him in his left eye.

It can be inferred from this that this individual was cared for during his lifetime by other members of his group, since without such help he would not have been able to survive to the age he did. Perhaps calling the attention he received 'medical' stretches a point but medicine does belong to a long tradition of caring which has taken some very different forms.

In pre-literate societies, disease is explained in a magico-religious context. Disease results from either 'object-intrusion' - that is, the intervention of some external agent, sometimes a spirit or demon - or it may result from personal 'spirit-loss' by the individual concerned. The prevailing notion is that living a 'good' life is the best guarantee of health - or at least of not being affected by untoward disease. However, even in modern, technologically advanced societies, essentially magico-religious views are sometimes voiced. That AIDS is divine retribution sent to punishment homosexuals is a fairly recent example.

Hippocrates (c460-380BC), however, did not separate disease from the person in this way. Disease was personal and resulted from an imbalance, an excess or a deficiency in that individual's 'humours'. Paracelsus (1493-1541) and later van Helmont (1579-1644) challenged the Hippocratic notion with the idea that disease was ontologically distinct from the affected individual. There was, they held, an ens (L. 'existing thing') that could invade a person or an organ.

A diseased state, therefore, differed fundamentally from a healthy one. Disease was the result of a bodily invasion by some separate thing. Just four weeks ago, one of my students used the expression "... when disease enters the body ..." thus echoing this same sentiment. A medically useful consequence of the Paracelsus/van Helmont notion was that disease could be classified systematically - based, that is, upon symptoms rather than causes. Thomas Sydenham (1624-1689) performed early attempts at such a classification, but the notion achieved added status when Linnaeus (Carl von Linné (1707-1778)) - famous for his 'Systema Naturae' (System of Nature - 1735) - produced a classification of diseases called 'Genera Morborum' (Types of Diseases - 1763).

There were other classifiers that could be named. For example:

· Boissier de Sauvages (1706-1767)

· William Cullen (1710-1790)

· Philippe Pinel (1745-1826)

· J.L. Schoenlein (1793-1864)

Of these, de Sauvages, for example, distinguished some 2400 different diseases that he grouped into families, orders, genera, species and varieties.

From the late eighteenth century, there came a move towards associating symptoms experienced in life with specific pathological changes visible post mortem. Giovanni Battista Morgagni (1682-1771) located the seats of disease in the organs and the work of François Xavier Bichat (1771-1802) took this down to tissue level - both of these were essentially anatomical approaches.

William Cullen (1710-1790) and John Brown (1735-1788) took a more physiological approach focusing, in particular, on the role of the nervous system. François-Joseph-Victor Broussais (1772-1838) introduced the idea that disease was related to dynamic patho-physiological processes. Although some of his ideas now appear rather odd to modern audiences, his was essentially a 'physiological medicine'. The physiology of Claude Bernard (1813-1878) and the cellular pathology of Rudolf Virchow (1821-1902) reinforced the notion of the physiological nature of disease. However, following the work of Louis Pasteur (1822-1895) and Robert Koch (1843-1910), the 1870s saw the emergence of the germ theory of disease that once again reinforced the idea of disease resulting from bodily invasion by external agents.

Perceptions of disease did change in the twentieth century. But we still live in the wake of past ideas and the ways in which some people speak and think reflects this. For example, the general, media-fed view now seems to be that every disease has to have a 'gene' associated with it. Even this has a lot of old thinking attached. Where a disease is caused by a so-called 'faulty gene', it is still perceived as being a disease caused by a separate 'entity' - in this case a 'gene' - even though that gene is integral to the person affected. It is not the person who is 'faulty' but something portrayed as being separate from them. Genes are often spoken of in the media as if they have a separate existence all of their own.

In examples such as these, Evolutionary Medicine is ideally situated to step in and influence our perception of disease for the better. It is Evolutionary Medicine that can give some explanation for the persistence and wider significance of such genes. But this belongs largely to the twenty-first century.

The history of medicine, therefore, is long and is, in effect, the history of people's individual experiential problems and the ways in which their fellows respond in order to help them. In so doing, there have been numerous, and sometimes fanciful, attempts at explaining the causal basis of what is going on. Explanation and treatment go together. Medicine, beginning with its magico-religious origins, has been offering explanations for human experience from well before the time when biology became a science in the modern sense.

When biology emerged as the study of life, around the turn of the nineteenth century, it did so into a world already coloured by medicine's attempts at explaining the problems of (human) life. To suggest that 'Biology is the basic science for Medicine' is not wrong but the influence of medicine on biology should not be overlooked either. Medicine does not rest on biology - the two interact and influence each other. Certainly, biology has aided the technical development of medicine but there are old notions, like disease being a separate entity that can enter an individual, which still permeate people's thinking and speech, however careful they may be to avoid such slips. Where medicine and biology meet, biology must speak with its own distinct voice and must avoid using notions inherited uncritically from medicine.

The terms 'disease' and 'health' may be two such inherited notions. They may have biological bases but they also come out of individual experience and have a host of non-biological trappings. The terms have medical usefulness but their 'biological meaning' may be something quite different.

There are significant contrasts between medicine and biology to be made, however. While sciences like biology tend to think that they operate in a value-free way, medicine does not and recognizes the fact. Biology deals with populations, their statistical norms and averages, whereas medicine, not withstanding the activities of epidemiologists and the move towards what is called 'evidence-based medicine', deals primarily with individuals. Biology is a science; medicine is often described as an art. Medicine has been seen as the profession most closely associated with disease and health, whereas biology has been more closely associated with the normal, that is, with species design and with the taxonomy of the typical. In biology, not only do we think of species as populations but sometimes only as gene pools. And at times, we even seem to forget that sexually reproductive species are composed of a male form and a female form each leading essentially different lives.

Applying evolutionary theory to medicine must inevitably bring to medicine the realisation that natural selection, which has shaped us and shaped our vulnerability to disease, is a process that acts without compunction. Those whom we would strive to help to survive; natural selection has no compassion for and, at times, positively acts to eliminate! Evolution operates one way; human nature (although itself a product of evolution) operates in quite another. By applying evolutionary theory to medicine, we are opening the door to the realization that we are shaped by processes that act without compunction or compassion. How such harsh realities as these are integrated into medicine may need careful consideration. There may even be potential for ethical problems to arise. Thus it may be a good time for ethicists to become more interested in Evolutionary Medicine.

The term 'Darwinian' tends to be associated with a certain way of looking at evolution. This way of looking is sometimes referred to as 'classical', 'traditional' or 'conservative' and tries to ascertain the genealogical ancestry of organisms so as to map out family trees. But there are other ways of looking at evolution and its products, for example, via the cladistic style of taxonomy. This approach seeks to comprehend the relationship between organisms - living and dead - by comparing a variety of morphological, physiological or biochemical characteristics. In so doing, cladistics attempts to make taxonomy a testable and objective operation. Although it can infer common ancestries for different species, it recognises the impossibility of certainty and emphasises relationship instead.

Evolution, as a process, can be approached from different intellectual perspectives. It is not the case that a given process, such as evolution, should have a single way of being comprehended. Indeed, it must surely be the case that each worker has his or her own different perspective on the subject. Often these differences are minor; sometimes they spark vigorous disputes. The late Stephen Jay Gould was frequently at the centre of such debates. Richard Dawkins, often at odds with Gould, has discussed the concept of the meme and also that of the extended phenotype. Both of these contribute to and influence organic evolution but are not part of the strictly 'classical' approach or way of thinking.

One is not, of course, suggesting that the current evolutionary paradigm, as a whole, needs replacing or that it needs significant alternation. But there may be other ways of understanding its richness. A species, such as our own, whose members demonstrate an awareness that can be communicated via abstract ideas, is not typical of life on Earth. Our species is one that can respond to the problems of existence such that the weak are not necessarily eliminated. For a very long time, it has not been the fit alone that have been able to survive within the human species.

According to Boyd Eaton and colleagues, we are physically Stone Agers in the fast lane of modern life - people whose bodies are better suited to a former diet and exercise regime but who are now living quite differently - and reaping the consequences. From the Palaeolithic until now, genetic evolution has been limited but memetic evolution has been considerable. Both are implicated in Evolutionary Medicine. We should be careful not to see Evolutionary Medicine as being simply about organic change according to the mathematical principles of evolutionary theory but as something more extensive - perhaps even qualitative as well as quantitative.

Although, as I have just said, the current evolutionary paradigm may not be wrong when it comes to humans, Evolutionary Medicine needs to ask, is the current evolutionary paradigm complete? In its general form, can it accommodate humans fully? Is there, perhaps, a special form or understanding of evolution that needs to be worked out for that atypical species to which we belong?

Concerning evolution by natural selection, Darwin's bulldog, Thomas Henry Huxley (1825-1895) once remarked "How stupid of me not to have thought of it before." The trouble is that the idea is so simple (according to Daniel Dennett, "seductively" so) that it can be too easily misinterpreted.

Occasionally one hears - as I did, on the radio a couple of months ago - people talk of "What evolution (or, What natural selection) has given us." But evolution and natural selection are not themselves entities that impart things. We have not been given things; we are those things: We are the products of - a process we call - evolution.

As I noted earlier, biology, in its modern form, is a relatively young science; the theory of evolution in its neo-Darwinian form is younger still. Thus the full force of what Daniel Dennett called 'Darwin's Dangerous Idea' has not yet been fully felt and has yet to truly permeate our understanding of ourselves. Only when our use of language in this area begins to change and people stop talking about 'Evolution' or 'Natural Selection' in the same way as some talk about 'Mother Nature' will this process even have begun to take hold.

Similarly, we still talk about 'having a disease' as if it were an entity separate from us. But although this might possibly be appropriate for an infectious disease - not withstanding the recent outbreak of Severe Acute Respiratory Syndrome (SARS) - this is no longer the predominant type of disease found in Western societies. The 20th century saw a shift from infectious to typically chronic conditions often associated with living too well and for too long. These conditions are typically associated with alterations in the very fabric of our own bodies.

Our language does not seem to accommodate this. There seems to be something wrong with talking about 'having a disease' when it is, in fact, what we are made of that is affected; when we are, in effect, physically 'at one' with the disease. There is no disease without a body to express it. And there is no disease without the capacity to become diseased. That body and that capacity are one and the same and have been produced via evolution.

In trying to understand the problems of disease and health and the role of biology in informing medicine, the more fundamental question 'What is Life?' should be remembered. Biologists once believed that this question could be answered but having found this to be seemingly impossible, the problem has failed to be passed on to their descendents.

By the early twentieth century, most biologists were uninterested in providing strong definitions of 'life'. Instead, they have come to focus more on the mechanisms involved. It has been left to certain physicists, among them Niels Bohr (1885-1962), Max Delbrück (1906-1981), but most famously Erwin Schrödinger (1887-1961) to address the question of 'What is Life?'

Back in the nineteenth century, however, Claude Bernard in his Leçons sur les Phénomènes de la Vie Communs aux Animaux et aux Végètaux (1878) suggested five general characteristics of living beings. These characteristics still prove informative today. These were:

· Organization

· Generation

· Nutrition

· Development

· Susceptibility to disease and death

What is noticeable is that the first four are characteristics that most of us might intuitively include, if we had to think hard enough, but the fifth - susceptibility to disease and death – would, I suggest, be overlooked completely by most. Thus, by implication, Bernard makes an important suggestion that understanding disease (and perhaps also death) has a place in understanding life.

The debate about 'disease' and 'health' that is currently up a blind alley has within it a subtlety. Is it a debate about how we use those words or is it a debate about the biological states to which those words are attached? It is not entirely clear at times. For me, what is important is the latter. It is not definitions of how we use the words 'disease' and 'health' that are important - although some consideration of this is necessary - but rather what is happening to the biological object.

It has been said of the French philosopher Georges Canguilhem (1904-1995) that his work sought to uncover a philosophy of life rather than a philosophy of biology, that is, he sought a philosophy of the object of biological study rather than a philosophy of the human enterprise called biology.

We do not have a true or fully developed 'philosophy of life' - a philosophy of the object of biological study - as yet. I am not aware of anybody working in quite that direction. Whatever forms a 'philosophy of life' might take, we can expect it to include a consideration of:

· Evolution - for that is the process by which life has come to be expressed in all its diversity.

But it might also include:

· Existence - what it is, biologically, 'to be' - and would include consideration of the phenomena of disease and death.

To that end, since one can only exist, or 'be', as an individual organism, a 'biology of the individual' is necessary. By this I mean a form of biology that recognises the ways in which the members of a species can be distinct from each other or even unique; a form of biology that takes account of individual variation and can accommodate those physical anomalies and differences that cannot justifiably be classified as abnormalities.

Just to be clear, the word 'existence' here has nothing to do with being conscious - and so is not immediately concerned with psychology. It is simply to do with an object being extant. Each of us is one such object - each with its own unique experience of life and its own unique vulnerability to disease. Here we are referring to biological objects that are quite unlike objects such as tables and chairs. We do not usually use the term 'object' for living things.

I believe that exploring the 'disease' and 'health' dimensions of existence has much to offer biologists but it must be done with due consideration to the fact that 'disease' and 'health' affect individuals not species, averages, populations or gene pools.

Most biological scientists interested in disease tend to be primarily concerned with the mechanics of pathological processes. Their interests lie mainly at the level of cells and tissues. But there are ramifications for the organism's 'existence' resulting from what is going on at those levels. By 'organism' and 'existence', I am not speaking exclusively about the human organism or human existence. Any plant or animal may be included here. Pathological changes at cell and tissue level can have a bearing upon the whole plant or whole animal. One may even suggest that changes at cell and tissue level have a 'meaning' for that organism - whether or not it is aware of it. This 'meaning' is the 'Biological Meaning' of my title. I could, perhaps, have used the phrase 'Biological Significance' instead. But as I said earlier, "I hope [that putting 'Biological' before 'Meaning' ...] leaves some unease about the phrase". [as] the words 'biological' and 'meaning' do not tend to go together." I wanted the 'unease' so that it would prove more thought-provoking and not slip into the usual biological way of thinking.

In the malaria-ridden areas of Central Africa, having what is sometimes called 'wild-type' (or, 'normal') haemoglobin puts one at risk of death by malaria, whereas, the sickle cell haemoglobin mutation "means" that individuals with sickle cell trait can survive better in such areas. But it also "means" that individuals with fully expressed sickle cell anaemia will die relatively young. In this instance, the "meaning" of a given genetic mutation is life for some and death for others. That "meaning" is ultimately expressed at the level of the organism - the level of the individual. The study of 'disease' and 'health' in terms of their "biological meaning" may be quite distinct from the study of specific pathological processes. The precise understanding of the pathological mechanisms involved in a certain condition may not be necessary to appreciate the '"biological meaning" of 'disease'' for a given organism, its survival and its reproductive potential.

Evolutionary Medicine has been described as '[the pursuit of] how evolutionary theory can sharpen the efficacy of medical practice' (Fabrega, 1997). I would like it to be seen as broader than that. Medicine has much to gain from the application of evolutionary theory but biology has much to gain from the enterprise, too. It should not be seen as a one-way process. However, at the moment the title Evolutionary, or Darwinian, Medicine may seem to favour an information flow towards medicine, rather than an information exchange.

At this early stage in Evolutionary Medicine's own evolution, it is important to recognise that the enterprise may be conceptually even richer than it currently appears and that much good can come the way of biology.

The discussion about 'disease' and 'health' is up a blind alley - there are no generally agreed definitions for either term. But medicine has been able to manage nonetheless.

I said earlier that, "Evolutionary Medicine would seem to need a concept of 'disease' or else it would be unclear what it was that we are trying 'to find evolutionary explanations for vulnerabilities to'." So far, Evolutionary Medicine has also managed without such definitions. But I have deliberately said 'would seem'; that "Evolutionary Medicine would seem to need a concept of 'disease'" knowing, all along, that it is probably the concept of disease that needs Evolutionary Medicine even more. In this way, we might understand 'disease' from a new and different perspective. Similarly, new light might also be shed on that state we call 'health'.

Evolutionary Medicine already offers a way of varying our axioms and presuppositions about disease and health within a medical context. I believe that it can also help us take a different view of disease and health within a biological context and that in so doing we can begin to understand life in different ways.

It may be possible that by exploring the biological meaning of disease and health, within an evolutionary context and as part of human existence, we may be able to develop a philosophy of 'life'. Or, in particular, a philosophy of 'human life'; a thoroughly objective understanding of the biology of human beings and the biology of human existence.

If this is indeed so, then Evolutionary Medicine will have an important role in shaping this understanding since it is Evolutionary Medicine - and Evolutionary Medicine alone - that currently connects evolutionary theory with individual human existence and an experience of life that characteristically includes susceptibility to disease and death.

(A lecture given at the invitation of the Konrad Lorenz Institute, at the Biozentrum, University of Vienna, 10th April 2003)