CONSTITUENTS OF MATTER 135 



something about the sun itself, which is ninety-three 

 million miles away ; but what we see is dependent upon 

 our eyes, and it is difficult to suppose that our eyes can 

 affect what happens at a distance of ninety-three million 

 miles. Physics tells us that certain electromagnetic 

 waves start from the sun, and reach our eyes after about 

 eight minutes. They there produce disturbances in the 

 rods and cones, thence in the optic nerve, thence in the 

 brain. At the end of this purely physical series, by some 

 odd miracle, comes the experience which we call " seeing 

 the sun," and it is such experiences which form the whole 

 and sole reason for our belief in the optic nerve, the rods 

 and cones, the ninety-three million miles, the electro- 

 magnetic waves, and the sun itself. It is this curious 

 oppositeness of direction between the order of causation 

 as affirmed by physics, and the order of evidence as 

 revealed by theory of knowledge, that causes the most 

 serious perplexities in regard to the nature of physical 

 reality. Anything that invalidates our seeing, as a source 

 of knowledge concerning physical reality, invalidates also 

 the whole of physics and physiology. And yet, starting 

 from a common-sense acceptance of our seeing, physics has 

 been led step by step to the construction of the causal chain 

 in which our seeing is the last link, and the immediate 

 object which we see cannot be regarded as that initial cause 

 which we believe to be ninety-three million miles away, and 

 which we are inclined to regard as the " real " sun. 



I have stated this difficulty as forcibly as I can, be- 

 cause I believe that it can only be answered by a radical 

 analysis and reconstruction of all the conceptions upon 

 whose employment it depends. 



Space, time, matter and cause, are the chief of these 

 conceptions. Let us begin with the conception of cause. 



Causal dependence, as I observed a moment ago, is a 



