of the equivalent transformability of the different forms of energy was too paradoxical in its simplicity to be immediately accepted. Rather have the three physicists to whom we are most indebted for the continued study of the law—Helmholtz, Clausius, and Lord Kelvin—believed that it must be interpreted as implying that all the different forms of energy are fundamentally the same—mechanical energy. In this way, what was regarded as most pressing—close connection with the prevailing mechanical conception of Nature—was reached; but a decisive side of the new thought was lost.
It required a half century for the idea to mature that this hypothetical addition to the law of energy did not give depth to the theory, but detracted from it on its most significant side—its freedom from all arbitrary hypothesis. And not even the recognition of this methodical circumstance, but the ultimate failure of all attempts satisfactorily to explain all the other forms of energy mechanically, has been, so far as our advance has as yet proceeded, the decisive reason for giving up the mechanical explanation.
You are impatient to learn how it is possible to form, by means of so abstract an idea as energy, a theory of the world that can compare in clearness and intuitiveness with the mechanical theory. I do not find the answer hard. What do we know of the physical world? Evidently only what our organs of sense permit to reach us from it. But what are the conditions under which these organs act? Turn things as we will, we find nothing common but that the sense organs react upon differences of energy between them and the surroundings. In a world the temperature of which is everywhere that of our body, we would know nothing of heat, just as we have no idea of the constant atmospheric pressure under which we live, and as we never gain knowledge of it till we establish a different pressure.
You will admit this, but you will not therefore give up matter, because energy must have a bearer. But I ask you, why? When all that we learn of the outer world are conditions of its energy, what ground have we for presuming anything in this same outer world of which we have never learned anything? Yes, I may be answered, energy is only something thought of, an abstraction, while matter is real. I reply: The contrary! Matter is a thing of thought, which we have constructed for ourselves, rather imperfectly, to represent what is permanent in the change of phenomena. Now that we begin to comprehend that the effective thing—that is, that which affects us—is only energy, we have to determine in what relations the two stand; and the result is indubitable that the predicate of reality can be ascribed only to energy.
