Our ancestors–probably long before they were even human–had to differentiate between living and non-living things. Living things can be eaten (and, importantly, can eat you back); non-living generally taste bad, can’t be eaten, and won’t try to eat you.
This is a task of such essential importance that I think it is basically an innate ability common to all thinking animals. Rabbits and fish need to distinguish between living things; both need to know whether the lump over there is a rock or a predator, after all. And we humans don’t have to explain to our children that cats and dogs are alive but tables aren’t. (Indeed, a defect in this ability that caused a person to regard tables as alive or other people as not is remarkable–and dangerous–when it happens.)
It is easy to divide most things into living and non-living. Living things move and grow; non-living things do…
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