[nyc111]

>We don't consider something "alive" if its behavior doesn't change when its environment does.

I think this is too vague. A cat may continue to sleep without responding to changes in its environment but may respond to an internal stimulus (hunger) and get up and go. So response or lack of response to the environment may not be a good criterion to define life.

[cobbzilla]

I think you're reading this too narrowly. The cat will respond to changes in its environment, even when sleeping. Try raising or lowering the temperature by an extreme amount while the cat is sleeping -- at some point it will awaken and seek a more comfortable place. To what extent does one need to change a virus's environment before it changes behavior? If its behavior is "fixed" by its genes and well understood, then there isn't much "dynamic" (complex) behavior being exhibited.

[dkersten]

So it’s more like “ability to respond to changes in the environment”.