The analogy of the question "What is life?" with "What is a computer?" bring up interesting parallels. The author defines a computer as any device that has transistors, RAM, etc that is, the computing substrate. But there were devices that didn't use transistors, but used vacuum tubes or mechanical gears (like Babbage's Analytical engine), which I think are still computers. We have some good theoretical model of computers: like Turing machines. One possible definition of a computer is any device that is Turing complete.

I wonder what such a theoretical model of life would look like.