our senses as deceiving us in the same way that the eyes do; and so makes 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, however, does it seem that Professor 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 criticisms.
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 misinterprets, it is desirable to give the right interpretation of it. On page 188, Professor 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 different future intervals of time." Now, the tacit implication here is, that I accept the doctrine of potential energy. The men of science named, with many others who might be added, hold that the total quantity of force remains constant. Against these it is urged that energy in becoming potential ceases to exist; and that, therefore, the doctrine is untrue. And being represented as holding 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 Professor 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 to ask how it happens that Professor Birks, elaborately criticising my views step by step, deliberately ignores the passages in which I have repudiated this doctrine? In the chapter on "The Continuity of Motion" I have, at considerable length, given reasons for regarding the conception of potential energy as an illegitimate one, and have distinctly stated that I am at issue with scientific