Page:Popular Science Monthly Volume 18.djvu/409

Rh 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 respecting the existence of any central force. Does Professor Birks contend that if I say that light, proceeding from a center, 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 that 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"? Professor 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 transforms 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 Professor Birks says it contains, I go on to show that I have not removed this passage because I have abandoned the belief it embodies. Clear proof is at hand. If Professor Birks will turn to the "Replies to Criticisms" contained in the third volume of my "Essays: Scientific, Political, and Speculative" (pp. 334-337), he will find that I have there defended the above proposition against a previous attack; and assigning, as I have done, justification for it, I have shown no sign of relinquishing it. Why, then, Professor Birks will ask, did I make the change in question? Had his mental attitude been other than it is, he might readily have divined the reason. Knowing, as he seemingly does, that this doctrine which he criticises had been already criticised in a similar manner (for otherwise he would scarcely have discovered the change I have made), he might have seen clearly enough that the passage was suppressed simply to deprive opponents of the opportunity of evading the general argument of the chapter by opening a side issue on a point not essential to its argument.

The chapter has for its subject, certain incapacities of the human mind—a subject, by the way, on which theologians are never tired of enlarging when it suits their own purpose, but on which an antagonist may not enlarge without exciting their anger. Various examples of these incapacities are given, to justify and enforce the conclusion drawn. Among these was originally included the example in