22 WHAT IS SCIENCE? 



information about the external world. In general we 

 have not the slightest difficulty in distinguishing such 

 sensations from other thoughts and other mental events ; 

 but there are exceptions to the general rule. Thus when 

 we awake from a very vivid dream there is often a con- 

 siderable interval in which we are not sure whether our 

 dream experiences were real ; we have been experiencing 

 perceptions so very like the sensations which tell us about 

 the external world that, if we were left to judge solely 

 on the basis of their mental quality, we should probably 

 think they were sensations and gave us information 

 about the external world. Doubtless we have all had so 

 many dreams and are so well aware of the circumstances 

 in which they are likely to occur that a very brief reflec- 

 tion is usually sufficient to enable us to decide whether 

 we were dreaming or were really hearing or seeing some- 

 thing. Nevertheless doubtful cases do arise ; and, when 

 they do arise, what do we regard as a certain test to 

 decide the matter ? Surely the test is whether anyone 

 else has had the same experience. If we suddenly awake 

 imagining that we have heard a banging at the front 

 door, and if there is somebody else in the room who 

 shows no sign of having heard anything, we conclude 

 at once that we were dreaming. But, if somebody else 

 also heard the noise, we have no further doubt that ours 

 was a real sensation. 



In the same way, but less frequently, people are some- 

 times subject to hallucinations when awake. If some- 

 body tells us that he has seen a transparent old gentleman 

 clanking chains about the passages and carrying his 

 head under his arm, our disposition to believe that he 

 has seen a ghost will doubtless depend largely upon our 

 attitude to the general question of the existence of 

 ghosts. But, whatever that attitude may be, our belief 

 would be enormously strengthened if we found that 

 other people present at the same time and place had 



