On this point there has been much confusion of ideas. When I put the question to Tyndall in conversation, nearly thirty years ago, he seemed to think that there must be some such completeness of correlation between the physical and the psychical; but his mind was not at ease on the subject. Herbert Spencer, in his "First Principles," rather cautiously took the same direction and tried to show how a certain amount of motion might be transformable into a certain amount of feeling. He observed that the consciousness of effort or muscular strain in lifting a heavy weight is more intense than in lifting a light weight, and that when a loud sound sets up atmospheric vibrations of great amplitude the shock to our auditory consciousness is correspondingly greater than in the case of a gentle sound which sets up vibrations of small amplitude. But when he comes to the inner regions of thought and emotion which are not reached by percussion and strain, he is less successful in finding illustrations. It is especially worthy of note that in the final edition of "First Principles," published in this year 1900 and in Spencer's eighty-first, he goes very far toward withdrawing from his original position, while in his Preface he calls attention to this change as one of the most important in the book. In my "Cosmic Philosophy," published in 1874, I maintained that to prove the transformation of motion into feeling or of feeling into motion is in the very nature of things impossible. In order to be convinced of this, let us go back a few years and ask how the great doctrine of the correlation of forces became established. Its first absolute verification occurred about 1846, when Dr. Joule showed "that the fall of 772 lbs. through one foot will raise the temperature of a pound of water one degree of Fahrenheit."[2] When this was proved it gave us the mechanical equivalent of heat, and the theory acquired a truly scientific character. Similar quantitative correlations were established in the case of heat and chemical action by Dulong and Petit, and in the case of chemical action and electricity by Faraday. The truth of the theory is wholly a question of quantitative measurement. Now you can measure heat, you can measure electricity, and since the action of nerves in all probability consists of undulatory motions it is to some extent measurable, and doubtless would be completely measurable had we the means. But when you come to thoughts and emotions, I beg to know how you are going to work to give an account of them in foot-pounds! It is not simply that we have no means at hand, no calculus equal to the occasion; the thing is absurd on its face. It is as true to-day as it was in the time of Descartes that thought is devoid of extension and cannot be submitted to mechanical measurement. [2] Herbert Spencer, _First Principles_ (final ed.), p. 185.