_The Idea of Energy Derived from the Kinetic Theory._—The notion of energy is not less clear than the notion of matter, it is only more novel to our minds. We are led to it by the mechanical conception which now dominates the whole of physics, _the kinetic conception_, according to which in the sensible universe there are no phenomena but those of motion. Heat, sound, light, with all their manifestations so complex and so varied, may, according to this theory, be explained by motion. But then, if outside the brain and the mind which has consciousness and which perceives, Nature really offers us only motion, it follows that all phenomena are essentially homogenous among one another, and that their apparent heterogeneity is only the result of the intervention of our sensorium. They differ only in so far as movements are capable of differing—that is to say, in velocity, mass, and trajectory. There is something fundamental which is common to them and this _quid commune_ is _energy_. Thus the idea of energy may be derived from the kinetic conception, and this is the usual method of exposition. This method has the great inconvenience of causing an idea which lays claim to reality to depend upon an hypothesis. And besides that, it gives a view of it which may be false. It makes of the different forms of energy something more than varieties which are equivalent to one another. It makes of them _one and the same thing_. It blends into one the modalities of energy and mechanical energy. For the experimental idea of equivalence, the kinetic theory substitutes the arbitrary idea of the equality, the blending, and the fundamental homogeneity of phenomena. This no doubt is how the founders of energetics, Helmholtz, Clausius, and Lord Kelvin understood things. But a more attentive study and a more scrupulous determination not to go beyond the teaching of experiment should compel us to reform this manner of looking at it. And it is Ostwald’s merit that, after Hamilton, he insisted on this truth—that the various kinds of physical magnitudes furnished by the observation of phenomena are different and characteristic. In particular, we may distinguish among them those which belong to the order of _scalar_ magnitudes and others which are of the order of _vector_ magnitudes. _The Idea of Energy derived from the Connection of Phenomena._—The idea of energy is not absolutely connected with the kinetic theory, and it should not be exposed therefore to the vicissitudes experienced by that theory. It is of a higher order of truth. We can derive it from a less unsafe idea, namely that of the _connection of natural phenomena_. To conceive it we must get accustomed to this primordial truth, that there are no _phenomena isolated_ in time and space. This statement contains the whole point of view of energetics. The physics of early days had only an incomplete view of things, for it considered phenomena independently the one of the other.