Life is a challenge to define in clear terminology because life is a process and not a pure substance.

However, our understanding of life has come a long way from Empedocles (430 BCE), Democritus (460 BCE), Aristotle (322 BCE), vitalism (17th Century), and hylozoism and panpsychism.

Excerpted from the very excellent and referenced Wikipedia article on Life:

Since there is no unequivocal definition of life, the current understanding is descriptive, where life is a characteristic of organisms that exhibit all or most of the following phenomena:

1. Homeostasis: Regulation of the internal environment to maintain a constant state; for example, electrolyte concentration or sweating to reduce temperature. 2. Organization: Being structurally composed of one or more cells, which are the basic units of life. 3. Metabolism: Transformation of energy by converting chemicals and energy into cellular components (anabolism) and decomposing organic matter (catabolism). Living things require energy to maintain internal organization (homeostasis) and to produce the other phenomena associated with life. 4. Growth: Maintenance of a higher rate of anabolism than catabolism. A growing organism increases in size in all of its parts, rather than simply accumulating matter. 5. Adaptation: The ability to change over a period of time in response to the environment. This ability is fundamental to the process of evolution and is determined by the organism’s heredity as well as the composition of metabolized substances, and external factors present. 6. Response to stimuli: A response can take many forms, from the contraction of a unicellular organism to external chemicals, to complex reactions involving all the senses of multicellular organisms. A response is often expressed by motion, for example, the leaves of a plant turning toward the sun (phototropism) and by chemotaxis. 7. Reproduction: The ability to produce new individual organisms, either asexually from a single parent organism, or sexually from two parent organisms.

A biophysical handle is:

...life is a member of the class of phenomena which are open or continuous systems able to decrease their internal entropy at the expense of substances or free energy taken in from the environment and subsequently rejected in a degraded form.

Because life is a process, it is no easy feat to determine when it begins or ends giving forth to great ethical angst when discussing abortion and euthanasia and the establishing of public policy and laws dealing with life and death situations.

Posted: October 4th 2010

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A complex system of chemical reactions, essentially. Physics explains the characteristics and behavior of elementary particles, such as quarks, leptons, etc. Chemistry explains the interactions of groups of elementary particles (i.e. atoms and molecules). Biology explains the emergent properties of complex systems of chemical interactions, specifically with reference to what we call life.

There are plenty of complex systems of chemical interactions that we don’t call life – even some that take place in living things, like the duplication of a DNA molecule. There are also some that blur the edges. For example, viruses don’t fit into the classical definition of life: “Living organisms undergo metabolism, maintain homeostasis, possess a capacity to grow, respond to stimuli, reproduce and, through natural selection, adapt to their environment in successive generations” (Wikipedia). Viruses don’t undergo any kind of metabolism and can’t reproduce themselves; they hijack their host’s cellular machinery to do it for them.

Life really is just a category of systems of chemical reactions. Any special meaning we give it beyond that appears to be purely philosophical.

Posted: October 3rd 2010

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