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The accident left David with a very unusual form of brain damage. If you scratched, pricked, or kicked him, he felt no pain. But if he saw a lot of yellow, tasted oak, heard an opera singer hit a high C, made an unintentional pun, or had one of several other apparently random experiences, then he would feel pain, sometimes quite acutely.
Not only that, but he did not find the sensation of this pain at all unpleasant. He didn't deliberately seek out pain, but he did not make any efforts to avoid it either. This meant that he did not manifest his pain in the usual ways, such as crying out or writhing. The only physical signs of David being in pain were all forms of involuntary spasm: his shoulders would shrug, eyebrows lower and rise in quick succession, or his elbows flap out, making him look like a chicken.
David's neurologist, however, was deeply sceptical. He could see that David no longer felt pain as he had before, but whatever David was now feeling when he saw "too much yellow," it couldn't be pain. Pain was by definition an unpleasant thing that people tried to avoid. Perhaps his brain damage had made him forget what the sensation of real pain felt like.
Source: "Mad Pain and Martian Pain" by David Lewis, in Readings in Philosophy of Psychology, 1980.
Baggini, J., The Pig That Wants to Be Eaten, 2005, p. 202.
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What do you think? What can pain possibly be if it is private and subjective, as seems to be the case for David? How can we ever know what pain really is? I'll be back on Friday with an answer that hopefully isn't too painful to produce. Not that you'd know anything about it.