The pleasure-pain reaction is generally appropriate for the animal, but you can see that for thought it is not. The criterion for coherent thought is that it is true and correct. But if you can get pleasure or pain from thought then coherent thought is no longer functioning. Rather, the criterion has become whether the thought gives pleasure or pain, consequently that thought becomes destructive. If thought can be determined by pleasure or pain, that's already the beginning of a lot of trouble. And we conditioned by that.
Source: Thought as a System, Pages: 49
Contributed by: Chris