Philosophy, Psychiatry, & Psychology

Pain Definition

From the first twinge, it feels as if pain does not require definition. The experience of pain is so immediate and urgent that it seems too basic, too ineffable, too self-evident for definition. From the first-person perspective, pain is so compelling and obvious, it seems to demonstrate its essence without need for the specification of necessary and sufficient conditions. Yet medicine must address pain in some systematic and effective manner. Pain must be measured and treated. To do so we must understand the boundaries of the pain experience and its relation to other medical phenomena. Indeed, defining pain may be uniquely important for medicine because pain is the subjective medical phenomenon. As scientific medicine tries to keeps its eye squarely on objective facts about the body, it keeps tripping on the inescapable subjectivity of pain.

Patrick Wall provides an excellent review of the recent types of pain definitions in his Textbook of Pain (1989). The first type are definitions based on the stimulus, such as "pain is the sensation provoked by injury." These definitions fail because they only define a stimulus which may or may not lead to a state of mind that is left undefined. They assume a fixed stimulus-response relationship that does not exist for clinical pain. It is important to note that a favorite of materialist philosophers also falls prey to this shortcoming: "Pain is the firing of C-fibers" appears to avoid the indeterminacy of subjective definition, but is based on bad science. Such firing is neither necessary (as in pain syndromes from CNS damage) nor sufficient (as when someone is under general anesthesia) for pain.

The difficulties encountered in articulating any adequate objective and sub-personal account of pain have been well described by Daniel Dennett (1982). Locating pain in a particular part of the nervous system runs the risk of "flagrant category mistakes by confusing the personal and subpersonal levels of explanation." We must, after all, explain how a brain process makes the individual [End Page 277] feel pain. We might ask whether a central nervous system correlate of pain could be discovered, e.g., through PET scanning, that could be definitive for pain? A thought experiment concerning conflict between criteria is revealing. If someone claimed they felt pain but did not show the pattern of CNS activation thought definitive for pain, which is wrong—the person or the test? Bridging the gap between the objective physiological causes of pain and the subjective psychological experience of pain remains an unsolved problem for medicine and philosophy. Pain may not be identical with a state of any particular part of the nervous system nor even with the momentary configuration of the nervous system as a whole.

The second type, definitions based on the behavioral outcome, also fail due to their assumption of a hard wired connection between stimulus and response (now behavioral rather than neuronal). Here, pain is defined in terms of its tendency to provoke withdrawal by the organism. Withdrawal can be provoked by other aversive experiences, e.g., nausea, so relief by analgesia has often been added to this type of definition. Much animal testing of analgesics is based on reflex withdrawal of this type. But defining pain in terms of its propensity to provoke reflex withdrawal runs afoul of the fact that context often shapes the organismic response to a noxious stimulus. The focus on reflex withdrawal is an attempt to avoid this variability or to force it into some obscure corner of the central nervous system.

The third type, definitions based on subjective experience, are, despite their vagueness, the most widely accepted today. Psychiatrist Harold Merskey first formulated in 1964 the definition adopted in 1979 as the official definition of pain by the International Association for the Study of Pain (IASP): "Pain is an unpleasant sensory and emotional experience associated...