Faced with the question ‘what is life?’ many scientists, and some philosophers, advance definitions of life. Defining life is especially popular among astrobiologists, many of whom are convinced that one cannot successfully search for truly novel forms of microbial life without a definition of life: How else will one recognize it if one encounters it? The extensive discussion of definitions of life in Part 2 (“Definition and nature of life”) of the CRC Handbook of Astrobiology (Kolb 2018) provides a salient illustration of this attitude. Along the same lines, a recent version of the NASA Astrobiology Strategy (Hays 2015) contains a large section devoted to “Key Research Questions for Defining Life” (p. 145).1 This chapter and the next explain why the scientific project of defining life is mistaken. Life is not the sort of thing that can be successfully defined. In truth, a definition of life is more likely to hinder than facilitate the discovery of novel forms of life.