In this study I came at once under the influence of a profound teacher--Johannes Müller; he who at the same time introduced E. Du Bois-Reymond, E. Brücke, C. Ludwig, and Virchow to the study of anatomy and physiology. As respects the critical questions about the nature of life, Müller still struggled between the older--essentially the metaphysical--view and the naturalistic one, which was then being developed; but the conviction that nothing could replace the knowledge of facts forced itself upon him with increasing certainty, and it may be that his influence over his students was the greater because he still so struggled. Young people are ready at once to attack the deepest problems, and thus I attacked the perplexing question of the nature of the vital force. Most physiologists had at that time adopted G. E. Stahl’s way out of the difficulty, that while it is the physical and chemical forces of the organs and substances of the living body which act on it, there is an indwelling vital soul or vital force which could bind and loose the activity of these forces; that after death the free action of these forces produces decomposition, while during life their action is continually being controlled by the soul of life. I had a misgiving that there was something against nature in this explanation; but it took me a good deal of trouble to state my misgiving in the form of a definite question. I found ultimately, in the latter years of my career as a student, that Stahl’s theory ascribed to every living body the nature of a _perpetuum mobile_. I was tolerably well acquainted with the controversies on this latter subject. In my school days I had heard it discussed by my father and our mathematical teachers, and while still a pupil of the Friedrich Wilhelms Institut I had helped in the library, and in my spare moments had looked through the works of Daniell, Bernouilli, D’Alembert, and other mathematicians of the last century. I thus came upon the question, ‘What relations must exist between the various kinds of natural forces for a perpetual motion to be possible?’ and the further one, ‘Do those relations actually exist?’ In my essay, ‘On the Conservation of Force,’ my aim was merely to give a critical investigation and arrangement of the facts for the benefit of physiologists.