If some form of Computational Logic is the language of human thought, then the best place to look for it would seem to be inside our heads. But if we simply look at the structure and activity of our brains, it would be like looking at the hardware of a computer when we want to learn about its software. Or it would be like trying to do sociology by studying the movement of atomic particles instead of studying human interactions. Better, it might seem, just to use common sense and rely on introspection.

But introspection is notoriously unreliable. Wishful thinking can trick us into seeing what we want to see, instead of seeing what is actually there. The behavioural psychologists of the first half of the twentieth century were so suspicious of introspection that they banned it altogether.