I believe that this discussion has begun to focus on the heart of the
matter. It appears that what is critical to the definition of life is
what living things do which things which are not living dont do.
Johnjoe McFadden has mentioned Aristotles reference to the ability to
initiate movement, but this remains vague. Literally, we see very
little movement in some living things at the macroscopic level and, if
we are required to rely on reports at the microscopic level: How did
Aristotle know they moved? AND What rate of movement is necessary? For
example, are crystals forming from solution alive? I think not.
Other participants have already acknowledged that neither replication or
reproduction can be part of a sensible definition because there are many
things which are alive which cant (or just dont) reproduce. The same
criticism holds for growth.
Recently, I have read material from researchers in AI who wish to claim
emergent properties as part of the necessary criteria. Inevitably, what
an emergent property is remains unclear. Its something like an
unpredictable synergistic effect of the interaction of subsystems. I
reject this because it renders life forever mysterious. If systems
interact, then there is no reason to suspect that we cannot study and
understand their interaction. (Although we should acknowledge that we
can't solve even a "simple" three-body-problem in Newtonian physics and
we've had three hundred years to work on it.)
Metabolism is really an important feature of life as we can conceive of
it. There is, however, the problem of defining metabolism. I propose
this definition: "Those processes that are required for an entity to
acquire the energy and appropriate structural substances that are
necessary for the entity as a whole to endure while maintaining active
thermodynamic disequilibrium with its environment." Its a bit of a
mouthfull, but I believe that it covers the ground and includes all
known (uncontroversial) instances of life. I think it has the advantage
of not being tied to traditional biochemical systems and it might open
the door to the inclusion of computer programs (perhaps together with
their hardware) as being alive. However, this would only be if the
ALifers can identify the nature of the appropriate structural substances
for their claimants.
The other requirement of life which has only peripherally entered this
discussion so far is boundary. All life maintains a semipermeable
membrane around it which allows only some molecules to pass through it.
This boundary is an outcome of metabolism but is at the same time (as I
have said) "
necessary for the entity as a whole to endure
has been an implicit part of the discussion for some time. It needs to
be more explicitly acknowledged.
The interaction of metabolism and boundary was first recognised (to the
best of my knowledge) by Maturana and Varela in "The Tree of
Knowledge". Worth a look for further discussion, although Im not ready
to support all of their speculations.
With metabolism and boundary, cells are alive, multicellular organisms
are alive, maybe organs are alive (James Grier Miller thinks they are,
but for different reasons).
Viruses are not alive. They dont maintain "
" with the environment. Outside the cell, they are like
Prions are not alive for the same reason. (However, the issue of their
being only protein is still subject to dispute.)
Interestingly, the definition I have given recognises that life can be
turned on and off. For example, some plant seeds may be so "dormant" as
to not qualify as alive. Yet they retain the potential for life.
I regret that this post is so long. I look forward to your comments.
Kevin W L Croft
Unit for the History and Philosophy of Science
University of Sydney, Australia
email: kcroft at server.blo.su.oz.au
Phone: 61 (02) 9351 7088
Fax: 61 (02) 9351 3636