SCIENCE AND NATURE 23 



experienced the same sensations. In fact, a very little 

 reflection will show that our recognition of the possibility 

 of dreams and hallucinations is based almost entirely on 

 the fact that there are circumstances in which the sensa- 

 tions of one person may not be shared by others ; dreams 

 and hallucinations are simply mental experiences which, 

 though almost indistinguishable by the percipient from 

 sensations, are distinguished from sensations by being 

 peculiar to the percipient and in not being shared by 

 others. The community of sensations is our chief and 

 final test that experiences are true sensations such as 

 give information about the external world ; if we apply 

 other tests, it is only because this chief test is not available, 

 and any other tests we may apply are based on the results 

 which we are accustomed to obtain with this chief test. 



OUR BELIEF IN OTHER PEOPLE 



But now we must inquire a little more deeply and face 

 a difficulty. We believe in the external world because the 

 sensations of other people agree with our own. But 

 what reason have we to believe that there are other 

 people ? In our discussion hitherto we have spoken 

 of the world as divided into two parts, man and nature, 

 and we have regarded the external world as the same 

 thing as nature. But it is not really the same thing. 

 If I divide the world into man and nature, you are not 

 part of nature ; but if I divide the world into an external 

 and an internal part, you are part of the external part. 

 " You " are not " me " and " I " am not " you," you are 

 part of my external world and I am part of yours. Nature, 

 the part of the external world that is not man, is the same 

 thing as that part of the world which is external to all 

 men ; but it is not the same thing as my external world 

 or as your external world. Accordingly, if I am asking 

 what evidence there is for an external world, I must first 



