598 APPENDIX. 



" So that the implication of uncertainty has infected the very word 

 appearance. Hence, Philosophy, by giving it an extended meaning, leads 

 us to think of all our senses as deceiving us in the same way that the eyes 

 do ; and so make us feel ourselves floating in a world of phantasms. Had 

 phenomenon and appearance no such misleading associations, little, if any, 

 of this mental confusion would result. Or did- we in place of them use 

 the term effect, which is equally applicable to all impressions produced on 

 consciousness through any of the senses, and which carries with it in 

 thought the necessary correlative cause, with which it is equally real, we 

 should be in little danger of falling into the insanities of idealism." 



This caution was intended for the general reader. That it 

 might be needed by one who should undertake to deal with 

 the work critically, never occurred to me. Not only, how- 

 ever, does it seem that Prof. Birks (who quotes the last three 

 words of the paragraph) needs such a caution, but it further 

 seems that the caution is thrown away upon him. For just 

 those misinterpretations of the words above pointed out, are 

 the misinterpretations he makes. After this I shall, I think, 

 be absolved from examining further his metaphysical criti- 

 cisms. 



Of his criticisms upon various of the physical doctrines 

 which this work contains, I will notice two only the one 

 because I wish to repudiate a view which, spite of abundant 

 evidence to the contrary, he ascribes to me; and the other 

 because, based as his statement is on a fact which he misin- 

 terprets, it is desirable to give the right interpretation of it. 

 On page 188, Prof. Birks says: 



" The Essence of the doctrine held by Mr. Grove, Dr. Tyndall, and Mr. 

 Spencer, and which the last has made the foundation of his whole theory 

 of Physical Fatalism, is that there is, every moment, an unchanging 

 total of Force, which never varies in amount, while it incessantly changes 

 its form. The Force, then, which persists, must be a present existence. 

 But Potential Energy is nothing of the kind. It is the sum of trillions 

 of trillions of future possibilities of force, ranging through trillions of 

 trillions of diiferent future intervals of time." 



Now the tacit inlplication here is, that I accept the doc- 

 trine of Potential Energy. The men of science named, with 

 many others who might be added, hold that the total quan- 

 tity of force remains constant. Against these it is urged that 

 energy in becoming potential, ceases to exist; and that there- 

 fore the doctrine is untrue. And being represented as hold- 

 ing this doctrine in common with them, I am said to have based 

 my general fabric of conclusions upon a fallacy. In the first 

 place I have to ask on what authority Prof. Birks assumes that 

 I hold the doctrine of Potential Energy in the way in which 

 it is held by those named? And in the second place I have 



