A great deal is known about life. Anatomists and taxonomists have studied the forms and
relations of more than a million separate species of plants and animals. Physiologists have
investigated the gross functioning of organisms. Biochemists have probed the biological
interactions of the organic molecules that make up life on our planet. Molecular biologists
have uncovered the very molecules responsible for reproduction and for the passage of
hereditary information from generation to generation, a subject that geneticists had previously studied without going to the molecular level. Ecologists have inquired into the
relations between organisms and their environments, ethologists the behavior of animals
and plants, embryologists the development of complex organisms from a single cell, evolutionary biologists the emergence of organisms from pre-existing forms over geological time.
Yet despite the enormous fund of information that each of these biological specialtieshas provided, it is a remarkable fact that no general agreement exists on what it is that is being studied. There is no generally accepted definition of life. In fact, there is a certain clearly discernible tendency for each biological specialty to define life in its own terms. The average person also tends to think of life in his own terms. For example, the man in the street, if asked about life on other planets, will often picture life of a distinctly human sort. Many individuals believe that insects are not animals, because by “animals” they mean “mammals.” Man tends to define in terms of the familiar. But the fundamental truths may not be familiar. Of the following definitions, the first two are in terms familiar in everyday life; the next three are based on more abstract concepts and theoretical frameworks.