(4) Is there a source of knowledge such as is believed in by those who appeal to “introspection”? According to what we have just been saying, all knowledge rests upon something which might, in a sense, be called “introspection”. Nevertheless, there may be some distinction to be discovered. I think myself that the only distinction of importance is in the degree of correlation with events outside the body of the observer. Suppose, for example, that a behaviourist is watching a rat in a maze, and that a friend is standing by. He says to the friend “Do you see that rat?” If the friend says yes, the behaviourist is engaged in his normal occupation of observing physical occurrences. But if the friend says no, the behaviourist exclaims, “I must give up this boot-legged whiskey”. In that case, if his horror still permits him to think clearly, he will be obliged to say that in watching the imaginary rat he was engaged in introspection. There was certainly something happening, and he could still obtain knowledge by observing what was happening, provided he abstained from supposing that it had a cause outside his body. But he cannot, without outside testimony or some other extraneous information, distinguish between the “real” rat and the “imaginary” one. Thus in the case of the “real” rat also, his primary datum ought to be considered introspective, in spite of the fact that it does not seem so; for the datum in the case of the “imaginary” rat also does not _seem_ to be merely introspective.