THE EXTERNAL WORLD 75 



must admit that we can know of the existence of realities 

 independent of ourselves. 



The question of causal dependence is much more 

 difficult. To know that one kind of thing is causally inde- 

 pendent of another, we must know that it actually occurs 

 without the other. Now it is fairly obvious that, whatever 

 legitimate meaning we give to the Self, our thoughts and 

 feelings are causally dependent upon ourselves, i.e. do 

 not occur when there is no Self for them to belong to. 

 But in the case of objects of sense this is not obvious ; 

 indeed, as we saw, the common-sense view is that such 

 objects persist in the absence of any percipient. If this 

 is the case, then they are causally independent of our- 

 selves ; if not, not. Thus in this form the question 

 reduces to the question whether we can know that objects 

 of sense, or any other objects not our own thoughts and 

 feelings, exist at times when we are not perceiving them. 

 This form, in which the difficult word " independent ' 

 no longer occurs, is the form in which we stated the 

 problem a minute ago. 



Our question in the above form raises two distinct 

 problems, which it is important to keep separate. First, 

 can we know that objects of sense, or very similar 

 objects, exist at times when we are not perceiving them ? 

 Secondly, if this cannot be known, can we know that 

 other objects, inferable from objects of sense but not 

 necessarily resembling them, exist either when we are 

 perceiving the objects of sense or at any other time ? 

 This latter problem arises in philosophy as the problem 

 of the " thing in itself," and in science as the problem of 

 matter as assumed in physics. We will consider this 

 latter problem first. 



Owing to the fact that we feel passive in sensation, we 

 naturally suppose that our sensations have outside causes. 



