24 WHAT IS SCIENCE? 



make up my mind whether you and other people are to 

 be regarded as part of it. ^If I do not regard you as part 

 of the external world, it is unreasonable to regard the 

 community of your sensations with mine as giving me 

 evidence of an external world. For that community 

 only gives me such evidence if you are external to me ; 

 if your sensations are internal to me, it is clear that the 

 fact that they agree with mine does not justify the 

 argument for the external world that we have just been 

 considering. On the other hand, if I know that you 

 are part of the external world, it is quite unnecessary 

 to examine your sensations and to inquire whether they 

 agree with my own in order to prove that there is an 

 external world ; for if there is not an external world, 

 you cannot be part of it. It seems that whichever 

 alternative I adopt, the argument for an external world 

 based on the community of your sensations and mine 

 breaks down. \ Either I must know already what the 

 argument professes to prove, or it provides no proofs. 

 Let us therefore examine rather more closely why we do 

 all actually believe that there are other people. 



Our reason for believing that there are other people 

 appears to be of this kind. There is attached to me a 

 portion of the external world that I call my body. It 

 is part of the external world because I can perceive it 

 by my senses ; I can see my own hand, just as I can see 

 any other external object ; I can hear my own voice ; 

 and with my hand I can feel my own eye. On the other 

 hand, I regard it is peculiarly attached to me, and as 

 "my" body, because it is very intimately under the control 

 of my will. I can move my hand and I can close or open 

 my eyes by simply desiring to so do ; it is much less 

 affected by obstinacy in the face of my desires than the 

 remainder of the external world. Now I know that 

 certain changes in this external object which I call r.iy 

 body, changes which I can perceive through my senses, 



