Ultimately it's a bit of an inprecise human concept. The boundaries of what fits in there might be somewhat unclear, but we definitely things that intuitively are (humans) and aren't (plants, rocks) in this set.

But if I cut myself, no amount of science can currently assess how much pain I feel or how much it bothers me.

You can observe that a human and a record player can both say "hello", but you can not make the argument from that that there is no way to disprove that a record player might wish to express a greeting to a fellow being.

A simple process can duplicate the outward appearance and effect of a complex one (an mp3 player can talk), and a complex process can duplicate the outward appearance and effect of a simple one (a human can crank a drive shaft), and neither of these means that one might just as well be the other. They don't mean anything at all by themselves either for proving or disproving.

Humans reacting to stimuli in largely similar ways to a plant, or even plain physical process like water filling a vessel or diffusion, neither proves nor disproves, nor even merely implies or suggests, nor even merely opens any doors to any room for doubts about anything.

It could be that there is no fundamental difference between a human and a plant and a toaster, but this observation about similar behavior provides nothing towards the argument.

You mean, incapable of understanding? Why would this be so?