In this pamphlet, which appeared in 1872, I made the first attempt to give an adequate exposition of my epistemological standpoint - which is based on a study of the physiology of the senses - with respect to science as a whole, and to express it more clearly in so far as it concerns physics. In it both every metaphysical and every one-sided mechanical view of physics were kept away, and an arrangement, according to the principle of economy of thought, of facts - of what is ascertained by the senses - was recommended. The investigation of the dependence of phenomena on one another was pointed out as the aim of natural science. The digressions, connected with this, on causality, space, and time, may then have appeared far from the point and hasty; but they were developed in my later writings, and do not, perhaps, lie so far from the science of to-day. Here, too, are to be found the fundamental ideas of the Mechanik of 1883, of the "Analyse der Empfindungen" of 1886, which was addressed principally to biologists, in the "Wärmelehre" of 1896, and in the "Erkenntnis und Irrtum" - a book which treats at length questions of the epistemology of physics - of 1905.Certainly it is right that, in response to repeated demands, this work, which was out of print twelve years ago, should appear in an unaltered form. I could not have entertained sanguine expectations as to the immediate result of my little work; indeed, many years before, Poggendorff had refused for his Annalen my short essay on the definition of mass, which definition is now generally accepted. When Max Planck wrote, fifteen years after I did, on the conservation of energy, he had a remark directed against one of my developments, without which remark one would have supposed that he had not seen my pamphlet at all. But it was a ray of hope for me when Kirchhoff pronounced, in 1874, the problem of mechanics to be the complete and simplest description of motions, and this nearly corresponded to the economical representation of facts. Helm esteemed the principle of the economy of thought and the tendency of my little treatise towards a general science of energetics. And, finally, though H. Hertz did not give an open expression of his sympathy, yet the utterances in his Mechanik of 1894 coincide as exactly as is possible with my own, considering that Hertz was a supporter of the mechanical and atomic physics and a follower of Kant. So those whose positions are near to mine are not the worst of men. But since, even at the present time, when I have almost reached the limit of human age, I can count on my fingers those whose standpoint is more or less near to my own - men like Stallo, W. K. Clifford, J Popper, W. Ostwald, K. Pearson, F. Wald, and P. Duhem, not to speak of the younger generation - it is evident that in this connexion we have to do with a very small minority. I cannot, then, share the apprehension that appears to lie behind utterances like that of M. Planck, that orthodox physics has need of such a powerful speech in its defence. Rather do I fear that, with or without such speeches, the simple, natural, and indeed inevitable reflections which I have tried to stir up will only come into their rights very late.