"The Need To Adopt A Rather Vague Definition Of Life" Reconsidered

Davies & Gribbin (1992: 280-1):

We have no difficulty in recognising life when we encounter it on Earth. Men, mice, mushrooms and microbes are all undeniably living. Yet what essential features do these systems display in common? Frequently cited properties of life are the ability to reproduce, response to external stimuli, and growth. The problem here is that other, manifestly inanimate systems also share these properties. Flames readily reproduce. Crystals both reproduce and grow into more organised structures. Bubbles respond to external stimuli by retreating when approached. 

Furthermore, once we probe below the level of our everyday experience — below the level accessible to our senses, especially those of sight and touch — there is no clear division, after all, between what is living and what is not. The classic example is the virus. In spite of the fact that viral diseases clearly involve biological activity, the virus itself does not even satisfy one of the criteria for life we have already mentioned — it cannot reproduce by itself, or with the aid of another virus. A virus can multiply only by invading a host cell and taking over its biochemical functions. In essence, it turns the cell into a production line for more viruses. It could be argued that under these circumstances the cell is no longer living, since it has lost the ability to reproduce itself. In isolation, though, viruses can be reduced to an inert dry powder, and differ little in their properties from other substances with less organised biological effects. 

These difficulties oblige us to adopt a rather vague definition of life.

Blogger Comments:

To be clear, an essential feature of life is metabolism: the set of life-sustaining chemical reactions in organisms. The absence of metabolism is symptomatic of the absence of life.