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INTRODUCTION
In her 1985 essay, Scarry [1] wrote “To have great pain is to have certainty; to hear that another person has pain is to have doubt.” This statement captures concisely the tension between subjective and objective approaches to this most universal of human—if not of all sentient beings—experiences. This tension pervades philosophical, scientific, clinical, societal, and legal discussions of the topic.
To discuss “pain” is immediately to encounter an aporia and to confront paradox. An aporia, from the Greek meaning, “lacking a passage, a path, or a way”, is “an expression of doubt, emerging when competing and compelling arguments are presented from both sides of a problem” [2]. Pain may be seen as an aporia, an entity that denies access to its secrets. Just when one feels that a concept of pain has been grasped, it slips away [3].
Nowhere is this more exemplified than by a number of paradoxes that pain presents. A paradox is a statement that leads to a seemingly self-contradictory or a logically unacceptable conclusion despite apparently valid reasoning from true premises. Perhaps the prime paradox of pain has been expressed best in two questions as formulated by Hardcastle [4]: 1. “How is it that pain is both a sort of experience but also the object of our experience?” 2. “What sort of thing is pain such that it is private, subjective, and infallible on the one hand, and yet refers to things in the world about which we can be mistaken on the other?”
These questions and similar others are the subjects of ongoing philosophical debate which, consistent with pain being an aporia, may be ultimately irresolvable. However, this article is not another attempt to confront that prime paradox but rather to expose a different set of paradoxes discoverable in the definition of pain promulgated by the International Association for the Study of Pain (IASP). That definition is, arguably, the most important contribution by and for the scientific and clinical worlds to frame the phenomenon of pain as an object of study, with major implications for health care delivery and public policy on health. On the basis of that definition, now in its second iteration [5], a conceptual framework for pain science and pain medicine has been established that has catalysed significant advancements in understanding the biological underpinnings of pain, including significant therapeutic developments, although perhaps not yet to the satisfaction of many sufferers or societies.
This article will argue that embedded in the IASP definition of pain are two previously unrecognised paradoxes that weaken its utility. The first is that it is possible for an experiencer and observer of pain to hold valid contradictory views; the second is the challenge posed by the construct of “nociplastic pain” which allows pain to be present in the absence of nociception (as currently defined). Taken together, the paradoxes mean that the definition simultaneously prescribes a connection between pain and tissue damage and allows that prescription to be violated. This is an untenable situation that has fundamental clinical significance beyond any philosophical interest. The resolution of this situation that is presented here is constructive and does not seek to resolve the fundamental paradox of pain itself but rather to strengthen the utility of the IASP definition for clinical and scientific purposes.
MAIN BODY
1. Revealing the paradoxes
1) IASP definitions of pain
In 1979, the IASP published its first iteration of the definition of pain: (A) An unpleasant sensory and emotional experience associated with actual or potential tissue damage or described in terms of such damage [6]. This definition was intended to be part of a taxonomic exercise. However, it became clear that, as a definition, it did not demarcate pain from non-pain but rather was an operational term for clinical practice [7]. The definition took into account “the fact that individuals tended to associate pain with damage to the body and that they tended to describe it in those terms, whether or not a lesion was present” [8]. Effectively, the construct of interest—pain—was translated into something related to it that was (potentially) observable, namely damage to the body [9].
Much commentary on the 1979 definition has been extensively summarised [5,10] and will not be reiterated here. In response to this commentary over 40 years, the definition was revised in 2020 to: (B) An unpleasant sensory and emotional experience associated with, or resembling that associated with, actual or potential tissue damage [5].
2) The definitional association of the experience of pain with tissue damage
Both the 1979 (A) and 2020 (B) definitions prescribe a connection—“associated with”—between pain and tissue damage. In effect, this grounds the subjective experience in the objective reality of tissue damage. However, “tissue damage” is not defined in the IASP lexicon, a point that will be discussed below. In effect, the only difference between these two definitions is the substitution of “resembling” for “described in terms of” with respect to the association with tissue damage.
However each of these definitions in fact consists of four non-identical sub-definitions. To use the 2020 definition, these are: (B-i) Pain is an unpleasant sensory and emotional experience associated with actual tissue damage. (B-ii) Pain is an unpleasant sensory and emotional experience associated with potential tissue damage. (B-iii) Pain is an unpleasant sensory and emotional experience resembling that associated with actual tissue damage. (B-iv) Pain is an unpleasant sensory and emotional experience resembling that associated with potential tissue damage.
The question then arises, is each one of these four experiences phenomenologically equivalent to the others? For example, do (B-i) and (B-iv) define the same experience, especially from the viewpoint of scientific enquiry and clinical application?
Sub-definition (B-i) is uncontroversial, as it reflects the common and common-sense-derived situation that a mutually recognisable experience labelled as “pain” often occurs when a body part is damaged, whether due to trauma or disease.
In logical terms, sub-definition (B-i) is equivalent to stating that a reported experience of pain implies that actual tissue damage has occurred.
3) The paradox posed by discordant views
This raises the important question of who determines whether there is an association with tissue damage (which is undefined) and who determines what it is to resemble that definitional association? Before discussing that question, the issue of “actual” and “potential” tissue damage needs to be addressed.
(1) “Actual” vs. “potential”
Actual tissue damage (sub-definition [B-i]) poses no problem when it is obvious. However what would be meant by “potential” tissue damage, that is, damage that might eventuate but has not yet occurred? Is “potential” tissue damage: ① a possible consequence of a stimulus that is currently not tissue-damaging but may become so, or ② a situation where tissue damage is imminent but has not yet occurred, or ③ an apprehension that tissue damage might occur? These examples reflect that “potential” in fact includes all possibilities and thus defies definition. The only relevant event then is actual tissue damage (which remains undefined).
It follows that the “actual or potential” qualification should be removed, so that a new formulation of the definition would refer only to “tissue damage,” with two sub-definitions: (C-i) Pain is an unpleasant sensory and emotional experience associated with tissue damage. (C-iii) Pain is an unpleasant sensory and emotional experience resembling that associated with tissue damage. This leads to consideration of what is meant by “resembling.”
(2) “Resembling”
“If they regard their experience as pain and if they report it in the same ways as pain caused by tissue damage, it should be accepted as pain” [6]. This note to the 1979 IASP definition of pain, often rendered as “Pain is what the person says it is,” appears to have laid down what is to be conveyed by the word “pain,” even more than the wording of the definition itself. The note to the 2020 definition is a little less generous: “A person’s report of an experience as pain should be respected” [5]. However, the recency of the 2020 note means that the 1979 note remains commonly quoted.
If “pain is what the person says it is,” then how would it ever be possible to deny an assertion of pain? A defeatist response would be, why would one even bother to attempt to define pain? Clearly, for scientific and clinical purposes, if not also for its moral, legal and philosophical implications, an attempt to define pain, despite the difficulties that attend an aporia, is necessary and desirable.
This issue of determining “resembling” comes down to the contrast between an experiencer’s view and an observer’s view of the phenomenon. Can an observer know what the other person is experiencing? In the ordinary sense of “knowing,” observers can and commonly do appreciate that another person is experiencing pain through observing the other’s language and/or behaviour and integrating that with their own experience and learning. This is the essence of the “mutually recognisable” qualifier of the pain experience as proposed recently [10]. However, an observer can never know with certainty whether the experiencer is making an association between their pain experience and tissue damage, as required by the definition.
In the situation (C-iii) above (pain is an unpleasant sensory and emotional experience resembling that associated with tissue damage), who is to determine the “resembling,” the experiencer or the observer? ① The experiencer might say, “It feels as if there is a knife in my back,” the implication being that tissue damage is occurring there. ② The observer might say, “On the basis of observing their behaviour, this person’s distress is as if there were tissue damage occurring in their back”.
But what if the observer—specifically a clinician—were to assert, “Despite it appearing that there might be tissue damage occurring in this person’s back, I can find no evidence thereof.”
Here is the first paradox. The experiencer maintains a belief that there must be tissue damage, while the observer can find no corroboration for that assertion. Although contradictory, both views are simultaneously valid.
Which view is to prevail? Suppose it is that of the experiencer, one of the principles laid down for the 2020 revision. In that case, there is neither a need nor a possibility for a definition of pain, as any subjective assertion qualifies—“pain is anything that hurts” [11].
A note that accompanies the 2020 definition states, “A person’s report of an experience as pain should be respected” [5]. However, “respecting” a report of a pain experience is not the same as accepting or believing it. In the case of this first paradox, the stipulation to respect a report is not violated at all, as the dialogue now becomes: ① Experiencer: “I (believe that I) have tissue damage (causing my pain).” ② Clinician-observer: “You do not have evidence of tissue damage, and therefore, your distress, which I respect, is not concordant with the accepted definition of ‘pain.’ We need to find another explanation for your distress.” It is argued, here and below, that a truly scientific resolution of this paradox can be determined only by allowing the observer’s view to prevail. That would privilege sub-proposition (C-i) over (C-iii).
4) The paradox posed by “nociplastic pain”
If sub-proposition (C-i) is the version that resolves the paradox of discordant views, there remains a challenge from the construct of “nociplastic” pain.
In 2016, the construct of “nociplastic” pain was developed, to account for patients whose pain could not attract either of the then current descriptors, “nociceptive” or “neuropathic,” but who nonetheless had clinical and psychophysical hypersensitivity to non-noxious stimuli applied to apparently normal tissues in the region of their pain [12]. The inescapable inference is that there has been a gain in nociceptive function in these patients.
Nociplastic pain has been formally defined as “pain that arises from altered nociception despite no clear evidence of actual or threatened tissue damage causing the activation of peripheral nociceptors or evidence for disease or lesion of the somatosensory system causing the pain” [12]. The construct of “nociplastic pain” has met with divergent receptions, ranging from over-enthusiastic adoption to outright rejection. The latter stance turns on the absence, in the definition of nociplastic pain above, of nociception as currently defined, namely a “neural process of encoding noxious stimuli” [13].
To the extent that nociplastic pain is not associated with “tissue damage causing the activation of peripheral nociceptors” or with “disease or lesion of the somatosensory system” (which could also qualify as “damage”), then “nociplastic pain” does not qualify as “pain.” However, at the same time, “nociplastic pain” does conform to the definition of pain, as in sub-definition (C-iii), to the degree that it “resembles” (C-i). This is the second paradox in the IASP definition, that “nociplastic pain” is simultaneously consistent with, yet contradictory to, the definition of pain.
2. Resolving the paradoxes
Unwittingly embedded in the IASP definition of pain is the paradox that it simultaneously prescribes a connection between pain and tissue damage yet allows that connection to be violated. The contradiction that constitutes this paradox is the inability to resolve the issue where there is discordance between the experiencer and observer of pain, in which the former relies on “resembling” the definitional connection while the latter relies on finding evidence for it.
1) Resolving the paradox of discordant views
So far, it has been identified that assertions such as “Pain is what the person says it is” and “If they regard their experience as pain and if they report it in the same ways as pain caused by tissue damage, it should be accepted as pain” lead to an untenable paradox of interpretation of the definition of pain when an experience “resembling” one associated with tissue damage (C-iii) is given weight equal to an experience that is associated with tissue damage (C-i).
The most straight-forward solution would be to use definition (C-i) above, as (C), pain is an unpleasant sensory and emotional experience associated with tissue damage, but with a new requirement that “tissue damage” be defined, which would also entail how that is to be determined, and therefore by whom.
This solution may be contentious as it challenges a principle that has been a basis for the IASP approach from the beginning, namely that of privileging the view of the experiencer of pain over that of the observer of pain. To have a definition that, in effect, makes it impossible to say that a reported experience is not pain serves neither taxonomic nor operational purposes. Proposition (C) above is equivalent to stating that the complaint of pain implies that tissue damage has occurred. In the clinic the onus of application of the definition then shifts onto the observer to determine that latter phenomenon. Put formally, that is to privilege the view of the clinical observer over that of the experiencer. However, as exemplified above, this in effect maintains respect for the experiencer’s point of view while adhering to the definitional connection between pain and tissue damage.
This point, which is likely to be controversial, actually raises the question of pain-related suffering. Although this article cannot do justice to a discussion of suffering and of pain-related suffering in particular, it can recognise the essential recent work that has been done to distinguish those two phenomena from the pain experience itself. Pain-related suffering has been defined as “a severely negative, complex, and dynamic experience in response to a perceived threat to an individual’s integrity as a self and identity as a person” [10], conceptualised as related to but separate and distinct from pain [12].
This has been expressed succinctly by Coninx [14]: “Chronic pain and suffering are two separate, albeit related experiences. Suffering, rather than pain, might be the driving factor for the distress and disability affecting many people in chronic pain ... Furthermore, suffering is not solely determined by the persistence, sensory intensity, or unpleasantness of (chronic) pain.”
Indeed, eight dimensions of pain-related suffering have been identified: physical, spiritual, existential, personal, social, cultural, affective, and cognitive [15]. These contributions clearly distinguish the somatic (or “biological”) dimensions of the experience of pain from the whole-person predicament.
Despite what may be seen as an affront to an experiencer, the stance entailed in proposition (C) above may enhance the clinician’s appreciation of the patient’s suffering. This further highlights the potential utility of having a definition of pain that avoids conflating that experience with suffering.
2) Resolving the paradox posed by nociplastic pain
The deliberations of the IASP Task Force that developed the revised 2020 definition (B) made it clear that the connection with tissue damage was not negotiable, even though “tissue damage” is not defined [5]. Therefore, proposition (C) does not deal with the challenge provided by the “nociplastic” construct. “Nociplastic” is part of the IASP terminology, which turns on the adjective “noxious,” which generally means “harmful” or “injurious.” In the IASP lexicon, “noxious” is defined only in the context of a “noxious stimulus” as one “that is damaging or threatens damage to normal tissues” [6].
There is no noun in English corresponding to “noxious.” It is argued here, therefore, that it is valid to extrapolate from undefined “tissue damage” to the biological event of signalling to the organism that a noxious (“damaging,” specifically “tissue-damaging”) event is occurring. This signalling is properly termed nociception, formally defined by the IASP as “the neural process of encoding noxious stimuli” [16].
Proposition (C) could then be recast as: (D) Pain is an unpleasant sensory and emotional experience associated with nociception. This maintains the connection in the definition by inserting a definable link—the signalling of tissue damage, namely “nociception”—between the undefined “tissue damage” and the pain experience. Thus, the intent of the definition is not compromised. Such a change in focus, from the externality of a noxious event to how the organism might respond to such an event, entails a more biologically based approach. However, to accommodate the clinical phenomena that lead to the construct of “nociplastic pain,” that approach requires a broader concept of nociception than the current “neural process of encoding noxious stimuli,” which considers nerve function only, towards that of activation of a nociceptive apparatus, reflecting the interaction of that neuronal substrate with cellular and humoral elements of the immune system. Both neuroanatomical [17] and physiological [13,18] bases for such an apparatus have been proposed.
The onus of application of proposition (D) remains with the observer to determine the presence of activation of the nociceptive apparatus. As exemplified above, this approach maintains respect for the experiencer’s point of view while adhering to the definitional connection between pain and nociception.
3) The issue of causation
The use of “associated with” in the current definition can be seen to hedge the question of whether the relationship of pain with tissue damage/nociception is causal. The note accompanying the 1979 definition of pain included the following: (a-i) “Activity induced in the nociceptor and nociceptive pathways by a noxious stimulus is not pain...” [6].
This has been stated more explicitly and simply in the notes to the 2020 definition: (a-ii) “Pain and nociception are different phenomena: the experience of pain cannot be reduced to activity in sensory neurons” [5].
Put another way, this means that nociception may not directly cause pain, which is consistent with experiential and experimental observation. To express that in causal language, nociception is not sufficient for pain.
The note to the 1979 definition [6] also stated: (b) “Many people report pain in the absence of tissue damage or any likely pathophysiological cause; usually (sic) this happens for psychological (sic) reasons;” (c) “This definition avoids tying pain to the stimulus”; and (d) “... [pain], which is always a psychological state, even though we may well appreciate that pain most often has a proximate physical cause” (Emphasis added here; note that, in the source [6], quotation (d) completes the sentence in quotation (a-i) above).
Points (b) and (c) together have led to the assertion that pain may occur in the absence of nociception or, in causal language, nociception is not necessary for pain.
However, points (c) and (d) above are contradictory. If (d) is taken as the definitive statement, then the “proximate physical cause” of pain that occurs “most often” can only be tissue damage or, preferably as argued here, nociception. Indeed the coincidence of nociception and pain is ubiquitous: not only is nociception associated with the vast majority of instances of pain but also nociception precedes pain, not the reverse. That is, a pain experience relies on the prior presence of nociception, even if pain does not always happen. In causal language, nociception is necessary for pain.
This inference, from the definition of pain itself, has been supported in a recent review, which has found no confident demonstration, clinically or experimentally, of pain experienced or induced in the absence of nociception. Therefore, it was concluded on both logical and empirical grounds that the assertion that nociception is not necessary for pain is untenable and should be replaced by “Nociception is necessary but not sufficient for pain” [19].
CONCLUSIONS
The revised (2020) IASP definition of pain [5] has been shown to contain two paradoxes, such that it simultaneously prescribes a connection between pain and tissue damage and allows that prescription to be violated.
This article has argued that this brace of paradoxes should be unacceptable in a definition that seeks to provide a basis for scientific enquiry and clinical practice. The IASP definition of pain never sought to resolve the prime philosophical paradox and does not, on its own, provide a taxonomical way of distinguishing “pain” from “non-pain.” Now, it has been shown to have failed in an operational role, as on the one hand it allows pain to be “what the person says it is,” while on the other hand it does not enforce its definitional connection with a biologically-based objective reality, be that tissue damage or nociception.
Therefore, for clinical and scientific purposes, and without claiming that it helps with the philosophers’ problem with pain, a first strengthening of the IASP definition can be proposed, to: (D) Pain is an unpleasant sensory and emotional experience associated with nociception, with the vital caveat that nociception be reconceptualised as activation of a neuronal apparatus that is capable of responding to a noxious stimulus.
However, considering the causal argument made above, that nociception is necessary for pain, it is no longer appropriate to continue to use the phrase “associated with” in the definition. Therefore, a second strengthening of the definition can be proposed, to: (E) Pain is an unpleasant sensory and emotional experience contingent on nociception, with nociception being reconceptualised as above.
Consistent with an earlier proposal [10], this proposition could indeed be modified further to: (F) Pain is a mutually recognised experience contingent on nociception, with the caveat as above.
Table 1 summarises the evolution of these proposals in this article. It is recognised that propositions (C) to (F) and their implications are likely to be controversial. However, this article has shown how this perspective can remove the current ambiguity of interpretation by grounding the pain experience in a reconceptualisation of nociception. This strengthens the definition of pain by making a clear distinction between that experience and nociception on the one hand and suffering on the other. This not only enhances the observer’s respect for the experiencer in the clinical encounter but also lays the foundation for distinct therapeutic approaches to those three elements.
Definitions do not only underpin clinical practice but also actively construct it, determining how pain is mutually understood, assessed and addressed in the consulting room. The clinical encounter itself depends on definitions, as every interaction involves fundamental assumptions about the relationship between subjective reports and objective findings, the legitimacy of personal experience, and the difficulty of dealing with the aporia of pain.
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