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Dimensions of Life Definitions

This chapter argues that the concept of “life” is used in several rather distinct ways: sometimes as an all-or-nothing phenomenon and other times as a matter of degree; sometimes referring to individual organisms and other times to communities; sometimes based on specific chemistries and other times on functions. In contrast to biologists in general, astrobiologists cannot take the status of their subject matter as living or nonliving for granted. There are at least two reasons to think astrobiologists need an understanding of what counts as life. The first is to set search criteria for finding “life as we don’t know it” in the universe. The second is to set success conditions conducive to agreement about when life has been found and when it has not. In addition to particular cases like the recent Mars finding by NASA, the meaning of “life” figures into a broader agenda in astrobiology: looking for biosignatures.