Both popular and scientific definitions of life must account for the possibility of the suboptimal operation of some function. Identifying the function in question and the criteria for optimality will be necessary steps in crafting a definition that is both intuitive and rigorous. I lay out a rule of thumb-the proper activity criterion-and a three-part typology of binary, range, and preference for understanding definitions of life. The resolution of "optimal" function within a scientific framework presents the central challenge to creating a successful definition of life. A brief history of definitions of life and explanations of biological function is presented to demonstrate the value of the typology. After analyzing three controversial cases-viruses, mules, and stars-I present three possible options for resolution: vitalism, reductionism, and instrumentalism. Only by confronting the consequences of each can we come to consensus about what is necessary and desirable from a common definition.
IntroductionThe primary challenge for generating a useful scientific definition of life comes from competing concepts of biological activity and our failure to make them explicit in our models. I set forth a three-part scheme for characterizing definitions of life, identifying a binary (presence or absence of some activity), a range (of operations for the activity), and a preference (for one end of the range). The three components together form a proper activity in biology (Table 1). To be clear, I am not proposing that proper activity be adopted as the best definition of life or even as a desirable definition for life. Instead, I am arguing that some notion of proper activity already exists within common scientific definitions. By making the implicit elements explicit, the notion can be analyzed to see whether it is useful and appropriate in the context of the biological sciences.The preference component has proved problematic historically. Prior to 1600, majority opinion was vitalist, holding that living things were ontologically different from the rest of the universe and governed by intrinsic, life-specific properties. Preference arose from internal sources, such as the soul. Over the next few centuries, modern science adopted mechanical explanations in place of vitalist explanations, replacing intrinsic properties with extrinsic universal forces acting on particles. A mechanical biology, however, was not embraced until the early 20th century, in part due to the apparent purposefulness of living things. I trace the Philos Theor Biol (2014) 6:e501 OPEN ACCESS -Freely Available at philosophyandtheoryinbiology.org MIX -PROPER ACTIVITY AND THE MEANING OF LIFE 2 OPEN ACCESS -Freely Available at philosophyandtheoryinbiology.org history of metabolic and evolutionary definitions of life over the last century as they attempt to make proper activities consistent with mechanism. I also look at attempts in the late 20th century to explain the concept of function by appeals to a proper activity. With this groundwork in place, I ad...