600 APPENDIX. 



that which Prof. Birks alleges. He represents me as assert- 

 ing " that gravitation is a necessary result of the laws of 

 space " (p. 227). I have asserted no such thing. He says 

 " There can be no a priori necessity that every particle should 

 act on every other at all at every distance " (p. 222). I have 

 nowhere said, or even hinted, that there is any such a priori 

 necessity. The notion " that gravitation results by a fatal 

 necessity from the laws of space/' which he ascribes to me 

 (p. 229) is one which I should repudiate as utterly absurd, and 

 one which is not in the remotest way implied by anything 1 

 have said. What I have said is that " Light, Heat, Gravita- 

 tion, and all central forces, vary inversely as the squares of 

 the distances," and that " this law is not simply an empirical 

 one, but one deducible mathematically from the relations of 

 space." Now- what is here said to be " deducible mathe- 

 matically from the relations of space?" Not a thing, or a 

 force, but a law. What is the law here said to be knowable 

 a priori? The law of variation of any or every central force. 

 And what is alone included in the assertion of this a priori 

 law? Simply this, that given a central force and such is the 

 law according to which it will vary. Nothing is alleged re- 

 specting the existence of any central force. Does Prof. Birks 

 contend that if I say that light, proceeding from a centre, 

 necessarily varies inversely as the square of the distance, I 

 thereby say that the existence of light itself is known a priori 

 as a result of space relations? When I assert that of the heat 

 radiating in all directions from a point, the quantity falling 

 on a given surface necessarily decreases as the square of the 

 distance increases, do I thereby assert the necessary existence 

 of the heat which conforms to this law? Why then do I, in 

 asserting tjiat the law of variation of gravity " results by a 

 fatal necessity from the laws of space " simultaneously assert 

 " that gravitation results by a fatal necessity from the laws 

 of space?" Prof. Birks, however, because I assert the first 

 says I assert the second. My proposition Central forces vary 

 inversely as the squares of the distances, he actually trans- 

 forms into the proposition There is a cosmical force which 

 varies inversely as the squares of the distances. And debiting 

 me with the last as identical with the first, proceeds, after 

 his manner, to debit me with various resulting absurdities. 



Having thus shown that the passage in question contains 

 no such statement as that which Prof. Birks says it contains, 

 I go on to show that I have not removed this passage because 



