Life

One of the oldest debates about the nature of life centered on whether living organisms functioned by means of a nonphysical process that lay outside material nature and therefore could not be fully understood by rational investigation or whether they could be understood in terms of everyday natural processes. The view that dominated the ancient and medieval worlds, known philosophically as idealism, claimed that living beings were qualitatively different from nonliving, representing a special set of categories whose "essence" existed only in the mind of the Creator. Associated particularly with the philosophy of Plato (c. 428–348 or 347 B.C.E.), this idealistic perspective claims that rational understanding of the essence of life is philosophically impossible, since by definition the categories of each unique species exist not in the material world but only in the nonmaterial, essentialist categories conceived by the Creator. Idealists did not deny the material reality of living organisms but only claimed that the essence of living organisms could never be understood by human investigation. Most idealists saw life as originating from a special, supernatural process of creation by a nonmaterial being.

The diversity of living organisms observed in the world was always viewed as a product of the creation of separate essences known as species, which were absolute and immutable. The biologist's role was to try to understand the essence as much as possible by examining individual representatives of the species and determining their common or essential features. Variation among individual members of a species was recognized of course but was viewed as natural deviations from the "essence" in the same way that any given piece of pottery can be viewed as a deviation from the potter's mold. The Platonic tradition thus became the basis for the Western idealistic view of "life" in the biological sense, informing questions not only about the functionality of organisms but also about their origin.

Idealism continued to form a backdrop to discussions of the nature and origin of species in the eighteenth and nineteenth centuries in the works of the taxonomist Carolus Linnaeus (Carl von Linné; the so-called "father" of taxonomy), the anatomist and paleontologist Georges Cuvier, and others who continued to see species as fixed entities formed by special creation. The "scientific creation" movement in the United States in the 1970s and 1980s and "intelligent design" arguments in the early twenty-first century are yet more manifestations of idealistic thinking, because they are based on the claim that creation by supernatural (nonmaterial) processes has occurred and is as theoretically valid as theories of descent with modification by material processes, such as gene mutation, selective agents of the environment, and differential fertility. "Intelligent design" is idealistic in that it postulates a supernatural, nonmaterial "designer" to explain the structure, function (adaptation), and diversity of organisms.

A second approach to understanding life, known as materialism, denies that living organisms have any special status in the physical world, maintaining that they are material beings, more complex than other entities in the universe but not immune to rational study. To materialists, all aspects of living organisms can potentially be understood by the same processes—known at present or knowable in the future—that govern all physical systems. Materialists have generally rejected all accounts of the origin of life by supernatural processes or nonmaterial "Creators." Historically the study of living systems has been characterized by the gradual retreat of idealistic in favor of materialistic approaches to understanding the nature of life.

Mechanistic materialism.

A long-standing debate among materialists has concerned whether and to what degree it is possible to treat organisms as simply special, complex kinds of machines or whether they are qualitatively different from machines, due to characteristics such as the ability to self-replicate or repair themselves, control their internal environment by self-regulating feedback loops, and so on. Mechanists argue that the basic principles on which machines function—matter in motion, transformation of energy, chemical reactions—are also at work in living organisms and provide a way of understanding life in accordance with the same laws of physics that govern nonliving systems.

Proponents of mechanistic thinking advocate the idea that complex entities are composed of separate, dissociable parts; that each part has its own characteristics that can only be investigated separately from other parts; that the functioning of the whole organism or machine is a result of the sum of its interacting parts and nothing more; and finally, that changes in the state of a machine or organism are the result of factors impinging on it from the outside (for example, machines and organisms decline in function due to physical wear and tear over time).

With the advent of the scientific revolution in the sixteenth and seventeenth centuries, living organisms came to be seen for the first time as truly mechanical entities functioning physically like machines and chemically like alchemical retorts. The "mechanical philosophy," as it was called, was a version of mechanistic materialism, describing organisms in terms of levers, pulleys, and chemical combustions. William Harvey (1578–1657) compared the animal heart to a pump, with valves to insure one-way flow; Giovanni Borelli (1608–1679) described flight in birds as the compression of a "wedge" of air between the wings as they moved upward; and René Descartes (1596–1650) described the contraction of muscles as due to a hydraulic flow of "nervous fluid" down the nerves into the muscle tissue. This view persisted through the Enlightenment, which made the mechanical analogy explicit in its obsession with "automata," models of birds, insects, and humans that moved by a series of windup gears and levers, drank from dishes of water, flapped their wings, or crowed.

In the nineteenth and twentieth centuries mechanistic views again gained considerable support with the school of Berlin medical materialists, spearheaded by the physicist Hermann von Helmholtz (1821–1894). In a famous manifesto of 1847, Helmholtz and his colleagues Ernst Brücke and Emil Du Bois-Reymond stated emphatically that living organisms have no special "vital force," and thus research on organisms should be based only on the known laws of physics and chemistry. Life was, to the medical materialists, a manifestation of matter in motion. Their successor in the next generation, the German-born physiologist Jacques Loeb, after moving to the United States published a new version of the materialists' manifesto as the widely read book The Mechanistic Conception of Life (1912). With a blatant mechanistic, materialist bias, Loeb declared that organisms moving unconsciously toward a light source were "photochemical machines enslaved to the light" and that life could ultimately be explained in terms of the physical chemistry of colloidal compounds. Though somewhat extreme, such claims emphasized that the biologist needed to probe "life" with the tools of physics and chemistry, not abstract or metaphysical conceptions.

Holistic materialism.

Opposed to the mechanistic view is a philosophy known as holism. While some forms of holism, especially in the early twentieth century, had a mystical, idealistic quality about them (associated in particular with Ludwig von Bertalanffy [1901–1971] and Jakob von Uexküll [1864–1944] in Germany and Pierre Teilhard de Chardin in France), holistic views within a materialist framework have become more and more common since the 1960s. The holistic materialist view maintains that while organisms (or any complex systems, for that matter) are indeed only material entities, they acquire special properties by virtue of their multiple levels of organization (from the atomic and molecular to the organismic and populational) and through the interactions of their parts.

What is missing from the mechanistic view, to holistic thinkers, is the description of each component of a system in terms not only of its isolated properties but also of its interactions with others. The characteristics derived from such interactions are known as "emergent properties" and function at a higher level of organization (including the parts and their interactions) than the individual parts alone. A cell can carry out certain functions in isolation (in a culture dish), but the many different functions it carries out as part of a tissue (a group of like cells) represent a higher level of organization. Individual nerve cells, for example, can depolarize when stimulated and release neurotransmitters at their terminal ends, thus acting like neurons; but when integrated into a nerve network, they function to stimulate a whole set of other neurons that can lead to complex outcomes, such as coordinated muscle contraction or thought, which would be emergent properties of the complex, integrated system. Holistic materialists do not admit supernatural or metaphysical explanations, only the insistence that complex systems are more than the sum of their individual parts.

Dialectical materialism.

A particular version of holistic materialism known as dialectical materialism emerged in the later nineteenth century in the work of Karl Marx (1818–1883) and Friedrich Engels (1820–1895) and was further developed in the twentieth century by Karl Kautsky (1854–1938) and Gyorgy Plekhanov (1856–1918) in the Soviet Union and J. B. S. Haldane (1892–1964) and others in Britain and subsequently by Richard Lewontin and Richard Levins (The Dialectical Biologist; 1985) in the United States. Dialectical materialists maintain not only that the whole is greater than the sum of its parts and that complex systems have various levels of organization, each with its own emergent properties, but also that such systems are always in flux, changing dynamically due to the interaction of opposing forces within them. Thus organisms move developmentally through their life cycle in a constant struggle between the opposing forces of anabolism (building up of molecules, tissues) and catabolism (the breaking down of molecules and tissues). Ultimately the forces of catabolism win out, and death follows. Similarly evolution can be seen as change in a species over time due to the interaction of the opposing forces of heredity (faithful replication) and variation (unfaithful replication). Constant temperature in homoeothermic organisms is maintained by the interaction of heat-generating and heat-dissipating processes. A dialectical materialist view of life particularly emphasizes the dynamic, ever-changing nature of living systems.

Holistic approaches to "life"—dialectical or otherwise—have become increasingly prominent in certain areas of the life sciences since the 1980s, for example, in physiology (especially the study of homeostatic feedback systems), in neurobiology (brain and behavior in particular), and in population biology and ecosystems work, where any useful understanding of the system must take into account numerous variables and their interactions. The advent of the computer in the study of such systems has aided greatly in providing ways of handling the immense amount of data that such investigations must utilize. Growing out of this revived holistic movement is an increasingly prominent field known as "systems science" or in some quarters as "the study of complexity."