Mechanism and Causality

The mechanist view of life, the view taken by physiologists, holds that all phenomena, no matter how complex, can ultimately be described in terms of physical and chemical laws. In contrast, vitalism is the view that some "vital force" beyond physics and chemistry is required to explain life. The mechanist view has predominated in the twentieth century because virtually all information gathered from observation and experiment has agreed with it.

Physiologists should not be misunderstood when they sometimes say that "the whole is greater than the sum of its parts." This statement in no way implies a vital force but rather recognizes that integration of an enormous number of individual physical and chemical events occurring at all levels of organization is required for biological systems to function.

A common denominator of physiological processes is their contribution to survival. Unfortunately, it is easy to misunderstand the nature of this relationship. Consider, for example, the statement, "During exercise a person sweats because the body needs to get rid of the excess heat generated." This type of statement is an example of teleology, the explanation of events in terms of purpose, but it is not an explanation at all in the scientific sense of the word. It is somewhat like saying, "The furnace is on because the house needs to be heated." Clearly, the furnace is on

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

A Framework for Human Physiology CHAPTER ONE

A Framework for Human Physiology CHAPTER ONE

not because it senses in some mystical manner the house's "needs," but because the temperature has fallen below the thermostat's set point and the electric current in the connecting wires has turned on the heater.

Of course, sweating really does serve a useful purpose during exercise because the excess heat, if not eliminated, might cause sickness or even death. But this is totally different from stating that a need to avoid injury causes the sweating. The cause of the sweating is a sequence of events initiated by the increased heat generation: increased heat generation ^ increased blood temperature ^ increased activity of specific nerve cells in the brain ^ increased activity of a series of nerve cells ^ increased production of sweat by the sweat-gland cells. Each step occurs by means of physicochemical changes in the cells involved. In science, to explain a phenomenon is to reduce it to a causally linked sequence of physicochemical events. This is the scientific meaning of causality, of the word "because."

This is a good place to emphasize that causal chains can be not only long, as in the example just cited, but also multiple. In other words, one should not assume the simple relationship of one cause, one effect. We shall see that multiple factors often must interact to elicit a response. To take an example from medicine, cigarette smoking can cause lung cancer, but the likelihood of cancer developing in a smoker depends on a variety of other factors, including the way that person's body processes the chemicals in cigarette smoke, the rate at which damaged molecules are repaired, and so on.

That a phenomenon is beneficial to a person, while not explaining the mechanism of the phenomenon, is of obvious interest and importance. Evolution is the key to understanding why most body activities do indeed appear to be purposeful, since responses that have survival value undergo natural selection. Throughout this book we emphasize how a particular process contributes to survival, but the reader must never confuse the survival value of a process with the explanation of the mechanisms by which the process occurs.

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