Pain is the only mode of experience that forces you to acknowledge its reality, but pain has no necessary objective correlate. You can inflict it, alter it, anaesthetise it, or in some cases even reverse its polarity, by actions on the body, but it is experience that verifies the body and not the other way around. We know what it means to have an unfelt pain, or a pain in a phantom limb, but without being able to explain it - this reaches far into the meaning of subjectivity.  There is nothing more private than pain, just as there is nothing more private than your awareness, but pain forces your response. It carves the world out of this dreaming stuff. Knowledge of the unreachable pain of others is the key to the depth of your world. The hard problem ought to be reframed as the problem of pain and then it would not stay enclosed within the imaginary bounds of a single organism.