Why is the definition of life so elusive? Epistemological considerations.
A central question of astrobiology concerns the origin and distribution of life in the Universe. For this reason, astrobiology can be considered to fall within the science called transitional biology. If we accept that life originated by a process of prebiotic chemical evolution, the next question concerns the nature of the transitional pathway from inanimate chemical systems to the first forms of life on Earth. These possible transitional states are the subject matter of transitional biology as a discipline. A related question concerns attempts to synthesize living systems in the laboratory. In contrast to traditional biology, the definition of life has an important methodological role in transitional biology, yet we still do not have a generally accepted definition despite the immense accumulation of empirical data in the life sciences. This review analyzes three approaches to defining life and concludes that all three are problematic in that they attempt to define life with undefined terms, confuse a description with a definition, or define life arbitrarily in terms of minimal living systems.