Definition of Life

The topic for today is part philosophical and part scientific. The philosophy of describing something as alive or not alive is a lot more grey than the scientific definition. Per Wikipedia, the scientific definition is:

"Living organisms undergo metabolism, maintain homeostasis, possess a capacity to grow, respond to stimuli, reproduce and, through natural selection, adapt to their environment in successive generations."

Lets break the definition down into its part to make it a little easy to understand.  "Living organisms undergo metabolism" means that the organism takes in energy in some form, converts it to a form which can be used and produces some sort of by-product, usually a waste.  The second part, "maintain homeostasis", just means that the organism has some sort of internal ecosystem separate from the exterior ecosystem in which the organism lives.  For a bacteria that is its cell membrane.  For humans it is our skin and external membranes (lining of the mouth and sinuses for example).  The third part, "possess a capacity to grow", ties in with the organism's metabolism, in that as it produces and consumes energy it will naturally become larger or more massive over time.  The fourth part, "respond to stimuli", can be given as a negative example, which is a rock.  Rocks do not respond to their environment except in a physical sense(such as water, wind, heat).  Living organisms, even bacteria, tend to move from environments which are unfriendly (that don't have necessary food, or have harmful toxins) to environments that are more friendly.  The last part, "reproduce and, through natural selection, adapt to their environment in successive generations", is necessary to the continuation of life and describes part of the process of evolution.

I believe this is the most controversial part of the definition.  I recently read a discussion relating to computer viruses and how they fit into the definition of life.  Many of the conclusions on assessing whether or not a string of computer code could be considered life got stuck on that last part of the definition.  My problem with using that as the basis to say computer code can never be alive is that the final part doesn't actually describe life itself, but rather a specific behavior.  An analogy would be for part of the definition of life that life always moves in a counter-clockwise fashion.  Anything that tends to move in a clockwise motion cannot be alive.  That is describing a behavior of the organism in the definition.

This is partly where the philosophy of describing life or "un-life" comes into play.  Many consider biological viruses to actually fit outside the definition of life.  On a technical level, digital viruses may not be the best candidate to look at when trying to point to an artificial form of life, however it will not be too long before artificial intelligence code becomes sufficiently advanced that it easily fills many criteria for even the scientific definition of life.