Life

Throughout recorded history human beings have recognized the qualitative difference between the living and non-living worlds, the animate and inanimate. Placing that recognition on solid, rational footing or giving it a quantitative basis has remained a major challenge, however. What exactly makes a living being so different from one that is nonliving? Living organisms carry out oxidation, for example, but so does a candle when it burns; living organisms grow from some sort of seedlike beginning to a larger form, but so does a crystal in a supersaturated solution; some organisms move, but others, like plants, do not; organisms reproduce (that is, make copies of themselves), but so do some molecules in the chemical process known as autocatalysis; viruses, probably the most confusing of all forms of matter in this regard, can enter living cells, reproduce themselves, break out of the cell, and infect other cells, yet they can also be crystallized and placed on a shelf for decades, only to become reactivated when placed in a solution in contact with host cells. In short, no single criterion or set of behaviors can unequivocally be said to distinguish living from nonliving matter. Yet there is also little doubt in most cases when we encounter a living organism. Life is characterized by a whole set of activities or functions, no one of which is unique but which collectively set living organisms apart from other physical entities.

In the history of Western thought, attempts to define life have been characterized by a series of alternative or dialectically opposed approaches that have reflected changing philosophical, cultural, and economic conditions. These alternative approaches will be described briefly and then applied to various aspects of the characterization of life.