No scientific definition of life has ever survived contact with real organisms

Zimmer walks through the history of proposed definitions — vitalist notions of a "life force," mechanist claims that organisms are just complex machines, and modern criteria like self-sustaining chemistry capable of Darwinian evolution (the definition NASA settled on in the early 1990s for astrobiology purposes). Every one of these definitions, he shows, immediately runs into exceptions once you go looking. A candle flame metabolizes and reproduces after a fashion; a growing crystal replicates its structure; neither is alive. Meanwhile mules can't reproduce and yet are obviously alive. The persistent failure isn't due to insufficient cleverness — it's structural, because "life" is a category humans invented to organize intuitions that don't cleanly map onto any single mechanism.

Takeaway: treat any confident one-line definition of life you encounter as a simplification, not a settled fact.