I ran into this question on Quora, a question-and-answer website, and thought it was interesting enough to be worth discussing in a blog post. At a superficial level the question might not seem very difficult. It would be easy to say that the answer is no, end of story. But that would be a facile response.
The reason why the question might superficially seem easy derives from a special feature of pain: the fact that for practical purposes it is a subjective phenomenon. When doctors want to know whether a patient is in pain, they have no better method of finding out than to ask a question, typically something like this: “On a scale of 1 to 10, where 1 is no pain and 10 is the worst possible pain, how would you rate your pain?”
There are lots of problems with that approach. Some people want to look tough and systematically understate their pain; others are whiny and complain about every twinge. People who aren’t fully conscious might not understand the question well enough to answer it. But the brute practical fact is that we don’t have anything better. Assessing pain is quite different from assessing, say, blood pressure. To measure blood pressure you attach a cuff, inflate it, and look at the reading on the guage. The resulting number is the patient’s blood pressure, and the patient’s own opinion has no bearing on the issue. But there is no corresponding approach for pain. There is no such thing as a pain meter. There is no instrument, at any level of sophistication, that will yield an objective reading of a patient’s pain level.
But the meaning of that fact shouldn’t be misconstrued. Our reliance on a purely subjective measure is really more indicative of the weakness of our technology than of the intrinsic nature of pain. We do know that pain has objective as well as subjective aspects: the problem is that our ability to measure the objective aspects is not adequate.
The root of the difficulty is that even though we think of pain as a simple unitary entity, it is actually more complex than that. In particular it has two crucial aspects, which might be termed sensory and evaluative. In its sensory aspect, pain is similar to other types of sensation, such as touch. But the special thing about pain is that it has a valence. Pain is unpleasant. The entity that we call pain is really a combination of sensation and unpleasantness. We think of them intuitively as inextricably linked, but scientific studies show that they are not. There are a number of ways in which the sensory aspect of pain can be dissociated from its unpleasantness.
On one hand, drugs such as heroin and morphine (opiates) reduce the unpleasantness of pain without greatly affecting its sensory aspect. People who use these drugs can still feel pain, but aren’t bothered by it as much. At the limit, there are certain types of brain damage that give rise to a syndrome called “pain asymbolia”, in which people are still capable of perceiving pain sensations but are not bothered by them at all.
In the opposite direction, people who are addicted to opiates and then deprived of them can be greatly bothered by pains that seem very minor at a sensory level. To an opiate addict, every little twinge is experienced as agonizing. It is possible that something similar occurs in fibromyalgia, where people report being in constant pain even though no physical source of pain can be detected. (Fibromyalgia is poorly understood, so that analysis might not be correct.)
If pain was purely subjective, then the only way to know anything at all about another person’s pain would be to ask about it. But obviously that isn’t the case: we are constantly inferring the presence of pain in other people without needing to interrogate them. If I encounter a man who has just broken his leg, and he is pale, his face is contorted in an expression of agony, and he gasps whenever anything touches the leg, then we are sure to infer that he is in pain. In fact, when it comes to pain in animals, that approach is the only one available to us, and very few people have doubts about its adequacy.
So now, with all that as background, let me propose a thought experiment. Suppose I come across a man with a broken leg who behaves just as described—to all appearances he behaves like he is in agony. But suppose when I ask him he denies feeling any pain, and shows no evidence of deliberate deception. Wouldn’t it be reasonable to say that he is in pain but doesn’t know it?
One possible response would be to say that nothing like that could ever happen. And maybe that’s right: I haven’t been able to find a case in the literature that fits the description I just gave. But I don’t think we can rule out the possibility. There are so many types of anosognosia (lack of awareness of deficit) that it seems unreasonable to rule out pain anosognosia. If it is possible for a person to be blind and not know it (as in Anton-Babinski syndrome), how can we assert without any doubt it is impossible to be in pain and not know it?