Commonsense tells us that pain is located in the physical limb in which it is felt, as though it occurs to a body part as an injury does. Children, in particular, view pain in this way. This chimes with pain's obvious function of drawing one's attention to an injury. However, given that people can experience pain without any apparent injury - as, say, in phantom limb pain -or injure themselves without feeling pain - such as through strong pain killers - this commonsense view seems illusory. Pain seems to be all in the head.
It's widely accepted that pain is a subjective and private experience. Firstly because only the individual in pain senses it. Hence, unlike the sight of an apple which can be seen by others, no one can literally feel an other's pain. Moreover, the very existence of pain depends on a person's awareness of it. This is certainly different from seeing an apple, the existence of which clearly doesn't depend upon one perceiving it.
Still, since one typically relates pain to a physical body part in time and space, how can pain be understood as an entirely subjective phenomenon, dependent on a subject's awareness for its very existence?This tension has intrigued philosophers over the ages.
Some philosophers hold that all perceptions, hallucinatory or 'real', are perceptions of projections of one's own mind. Thus, as one projects redness, roundness, smooth texture, etc, onto one's mind's 'screen' when imagining an apple, seeing an apple with the eyes also brings the mind to project redness, shape, texture, etc, onto its 'screen', creating a mental counterpart of the apple in the external world. It is only these mental counterparts of reality that we ever perceive. This explains why we often err in our perception of things: our eyes look at a red ball; our mind projects an image of an apple.
In this scheme, pain, like all perceptions, is a projection of the mind. It is unique in that the subjective nature of its perception is more obvious. Thus, when looking at an apple we're normally unaware that we're really perceiving our own mental counterparts of it. In pain, however, the dependence of the perception on one's awareness is more noticeable. According to these philosophers, the subjective/objective tension mentioned earlier spans across all perception, pain just helps bring the paradox of perception to light.
The above theory aside, is pain comparable to other sensory perceptions like seeing, hearing, or touching, where one experiences objects outside the mind? Perhaps pain offers us perception of the body's tissue damage, potential damage, type of damage, or the like?
Whereas perception through other senses is vulnerable to a mismatch between appearance and reality (one hears a voice which is really wind), pain is apparently immune to such incongruity: if one feels pain, one is definitely in pain. Pain thus differs from other sensory perceptions.
In defense, some argue that since pain merely carries the message of tissue injury (as reflected light carries the image of an object for visual perception), though one cannot be mistaken about experiencing pain, one can be mistaken about the body part that pain is a messenger of: we may think we've injured our hand due to pain signals but in actuality we haven't.
Yet, another problem exists. In normal sensory perception one typically focuses on aspects of the perceived object, its colour, shape, texture, etc. Concerning pain, however, one chiefly perceives the 'feeling' of pain itself rather than the nuances of the object its supposed to help us perceive, the damaged tissue that is.
Some say that pain differs to other senses in this respect because pain is negative and signals danger. It thus impels one to focus primarily on it in order that one attempts to alleviate it and thereby increase one's chances of survival.
But another question surfaces. Relative to other sensory perceptions, pain is strongly entwined with emotion. When seeing, touching, or smelling something, we do not necessarily experience any distinctive emotional arousal or response. And, even when we do, the emotion seems to be a secondary effect of the sensory perception. Pain is different: a feeling of dislike and desire for relief seem intrinsic to it.
Indeed, some philosophers don't classify pain as a 'cognitive' sense, a sense that can be separated from feelings and evaluations. Rather, they view it, and its positive counterpart, pleasure, as feelings of good or bad, positive or negative experiences.
Others, however, claim that the reactive-emotional component of pain is not intrinsic to pain. For instance, lobotomy patients, those on morphine, among others, may experience pain without being bothered by it at all. The pain perception is present but unaccompanied by the typical negative emotional reaction toward it.
It may be argued that still, in 99 percent of cases the emotional reaction does occur, which is atypical of other perceptual experiences. Furthermore, perhaps the 'pain' without the emotional- reactive element cannot even be classified as pain at all; who knows?
To conclude: the exact classification of pain remains just that: a pain!