[taneq]

People have always started with "I know it when I see it" and worked backwards to find a set of rules that fit the things they currently knew about. For instance, the requirement for life to be cellular was added when we realised that our previous definition of life didn't exclude fire.

Once robots tick all of our current boxes, I'm sure we'll add something like "must use a combination of electrical and chemical signalling" or "must be powered largely or wholly by ATP reactions."