But the truth is that the word Pain has two senses which must now be distinguished. a. A particular kind of sensation, probably conveyed by specialised nerve fibers, and recognisable by the patient as that kind of sensation whether he dislikes it or not (e.g., the faint ache in my limbs would be recognised as an ache even if I didn't object to it). b. Any experience, whether physical or mental, which the patient dislikes. It will be noticed that all Pains in sense a become pains in sense b if they are raised above a certain very low level of intensity, but that Pains in the b sense need not be Pains in the a sense.
C. S. Lewis, The Problem of Pain, Collier (New York: 1962) p. 90.