The main principles on which the present work is founded were broadly outlined in the author's _Terrestrial Energy_ in 1883, and also in a later paper in 1892. The views then expressed have since been amply verified by the course of events. In the march of progress, the forward strides of science have been of gigantic proportions. Its triumphs, however, have been in the realm, not of speculation or faith, but of experiment and fact. While, on the one hand, the careful and systematic examination and co-ordination of experimental facts has ever been leading to results of real practical value, on the other, the task of the theorists, in their efforts to explain phenomena on speculative grounds, has become increasingly severe, and the results obtained have been decreasingly satisfactory. Day by day it becomes more evident that not one of the many existing theories is adequate to the explanation of the known phenomena: but, in spite of this obvious fact, attempts are still constantly being made, even by most eminent men, to rule the results of experimental science into line with this or that accepted theory. The contradictions are many and glaring, but speculative methods are still rampant. They have become the fashion, or rather the fetish, of modern science. It would seem that no experimental result can be of any value until it is deductively accommodated to some preconceived hypothesis, until it is embodied and under the sway of what is practically scientific dogma. These methods have permeated all branches of science more or less, but in no sphere has the tendency to indulge in speculation been more pronounced than in that which deals with energetics. In no sphere, also, have the consequences of such indulgence been more disastrous. For the most part, the current conceptions of energy processes are crude, fanciful, and inconsistent with Nature. They require for their support--in fact, for their very existence--the acceptance of equally fantastic conceptions of mythical substances or ethereal media of whose real existence there is absolutely no experimental evidence. On the assumed properties or motions of such media are based the many inconsistent and useless attempts to explain phenomena. But, as already pointed out, Nature has unmistakably indicated the true path of progress to be that of experimental investigation. In the use of this method only phenomena can be employed, and any hypothesis which may be formulated as the result of research on these lines is of scientific value only in so far as it is the correct expression of the actual facts observed. By this method of holding close to Nature reliable working hypotheses can, if necessary, be formed, and real progress made. It is undeniably the method of true science.