Cumulative Record. Chapter 20: What Is Psychotic Behavior? Quote 19

It is easy to teach a child to distinguish between colors by presenting different colors and reinforcing his responses as right or wrong accordingly, but it is much more difficult to teach him to distinguish between different aches or pains, since the information as to whether his responses are right or wrong is much less reliable. It is this limited accessibility of the world within the skin, rather than its nature, which has been responsible for so much metaphysical speculation. (p. 317)