I’m getting convinced that pain, or even physical pain, is a category that does not cut nature at the joints. Consider that the taste of mouldy bread is more unpleasant than a typical vaccination jab, but the jab is physically painful while the taste is not. So neither are all physical unpleasantnesses pains, nor is it even the case that pains are distingushed from other unpleasantnesses by intensity. (That said, it is likely true that among physical sensations, the very most unpleasant ones are all pains.)
So, pains seem to be a subtype of displeasures. Is it a subtype that cuts nature at the joints? If we could say that pain just is tactile displeasure, then that would help. But not all tactile displeasure is pain. If I have a calloused hand and I move it against a fuzzy cloth and feel the callouses catching on the cloth, that’s definitely unpleasant—but not the least painful. So only some tactile unpleasantnesses are pain.
What unifies the tactile unpleasantnesses that are pains? Is there a common feel that all the pains have and that the other unpleasantnesses don’t? I doubt it. A burning pain and a dull ache feel quite different, and what they have in common appear to be (a) their tactile nature and (b) their unpleasantness. But (a) and (b) do not distinguish pains from other tactile unpleasantnesses.
I suspect that pains are like fish, which are a paraphyletic group. (Coelacanths and lungfish are more closely related to humans than to tuna.) They do not mark a natural kind of sensation. There are many kinds of unpleasant tactile sensations, such as itch, excessive cold, unpleasant roughness, burning pain, stabbing pain, dull achy pain, etc., and the ones that we call “pain” have nothing special in common.
This point is even clearer if we do not limit ourselves to physical pain. For the pain of embarrassment is, if anything, closer to disgusting taste than to stabbing pain.
All this suggests to me that pain is not a useful philosophical category. Unpleasantness or displeasure is a useful category. Specific types of unpleasantness, some of which are pains, are also useful categories. But pain as such is not.
