THE LAST RIDDLE 449 



This was a great stumbling-block to the acceptance of the 

 new gravitation theory. It involved the idea of action at a 

 distance. This is one of the inconceivables. No one saw this 

 more clearly than Newton himself. He had a highly speculative 

 mind that is to say, a very vivid imagination, scientifically 

 trained. He tried to think out a method by which this ap- 

 parent attraction could be accounted for. No abler head ever 

 pondered the subject. It is of interest to know precisely '.how 

 he conceived the problem. 



To the end of his days Newton refused to consider gravitation 

 as an inherent property of matter. So far as his physical specu- 

 lations were concerned, he was a thorough materialist. But as 

 late as 1717, in the second edition of his Opticks he wrote : 



" At the end of the Third Book I have added some Questions. 

 And to shew that I do not take Gravity for an essential Property 

 of Bodies, I have added one Question concerning its Cause, 

 chusing to propose it by way of a Question, because I am not 

 yet satisfied about it for want of Experiments." 



This same thought pervades the Principia. At the very 

 beginning of the work, as his eighth Definition, he says : 



" Wherefore the reader is not to imagine that by these words 

 I anywhere take upon me to define the kind or the manner of 

 any action, the causes or the physical reason thereof, or that I 

 attribute forces, in a true and physical sense, to certain centres 

 (which are only mathematical points) when at any time I happen 

 to speak of centres as attracting or as endued with attractive 

 powers." 



As Newton was the first to grasp the idea of attraction, so 

 was he the first to speculate upon its cause. At the end of 

 the Principia we find this pregnant paragraph : 



" Hitherto we have explained the phenomena of the heavens 

 and of our sea by the powers of gravity, but have not yet assigned 

 the cause of this power. This is certain, that it must proceed 

 from a cause that penetrates to the very centres of the sun and 

 the planets, without suffering the least diminution of its force ; 

 that operates not according to the quantity of the surfaces of 

 the particles upon which it acts (as mechanical causes used to 

 do) but according to the quantity of the solid matter which 

 they contain, and propagates its virtue on all sides to immense 

 distances, decreasing always in the duplicate proportion of the 

 distances. Gravitation towards the sun is made up out of the 



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