I strongly feel that life is a process, and should be defined in terms of what it does, not what it's made of. But adopting a process-based definition of life means we must consider whether our definition applies to entities that we think of as non-biological. For example (as my dad pointed out upon reading my previous post) crystals replicate their structure. Should they be considered alive? Computer viruses reproduce en masse; are they alive? Are "25 things about me" Facebook posts alive?
The example of crystals, more than anything else, convinces me that although reproduction is the central feature of life, it is not sufficient for defintional purposes. Crystals have not changed their nature since they first appeared on the earth. Their abitily to "adapt" is limited to conforming to the shape of their environment; they have produced no novelty in their billions-year history.
A glance at the wikipedia page on this subject gives us several other criteria we may wish to include, such as homeostasis (the ability to regulate one's internal evironment), and metabolism (the ability to convert raw materials to energy.) But these also seem peripheral to what I would consider the principal feature of life: its abilty to create new innovations in the world. These innovations are the product of evolution, so (as faithful reader samineru suggested), we should focus on entities that don't just reproduce, but evolve.
Evolution requires that:
- Entities compete for the chance to reproduce, based on their characteristics.
- Whenever an entity reproduces, its characteristics are passed on to its offspring, with slight variation.
These requirements rule out crystals: It would be hard to say that crystals are competing in any meaningful sense, and variations in the structure of a crystal are not (as far as I know) passed down onto other crystals seeded by it. "25 things about me" posts also don't compete, and small innovations in one post are not typically passed down into the other posts inspired by it. So neither crystals nor facebook memes are alive by this definition.
Computer viruses are more tricky. I don’t know of any viruses that mutate and pass mutations on when they spread. But one can imagine this happening in the near future: viruses producing their own innovations and finding ever more devious ways to infect other computers. If they did, would we call them alive? By this definition, we would.
Such questions are central to the field of artificial life. Artificial life (or “alife”) researchers write computer programs in which entities compete and evolve according to abstract sets of rules. There is debate in this community over whether they are actually creating life with these programs, or merely simulating it.
Many other intelligent people have written on the definition of life; I recommed these articles for further reading.