This decisive side of the new theory will come more plainly into view if I present the concept in a brief historical sketch. We have already seen that the progress of science is marked by the discovery of more and more general invariants; and I have shown how the first of these unchangeable entities, mass, has expanded into matter—that is, mass endowed with volume, weight, and chemical properties. Yet this idea was obviously insufficient from the beginning to cover the incessant variability of phenomena; and since Galileo's time, that of force has been added as a means of accounting for it. Yet force was destitute of the property of unchangeableness; and after functions were found in mechanics, in living force, and work which manifested themselves as partial invariants, Mayer discovered in energy the most universal invariant, dominating over the whole domain of the physical forces.
Conformably to this historical development, matter and energy continued existing together, and all that we could say of their mutual relations was that they mostly appeared with one another, or that matter was the bearer or the vessel of energy.
Are matter and energy, then, really different from one another like, for instance, body and soul? Or is not rather what we know and express of matter already comprised in the idea of energy, so that we can represent all phenomena with this one entity? In my opinion the answer can not be doubtful. What is included in the idea of matter is, first, mass—that is, the capacity for momentum; next, volume, or space-filling energy; then gravity, or the peculiar kind of static energy which is manifested in general weight; and, lastly, chemical properties, or chemical energy. We still have only energy; and if we think on about these various kinds of matter, there is nothing else left us, not even the space it occupies, for this, too, is knowable only by the application of energy, which is required to compress matter within it. Hence matter is nothing but a spatially contiguous group of different energies, and all that we can predicate of it we predicate only of these energies.
What I am at pains to assert here is so important that you will pardon me if I try to approach the subject more closely from another side. Permit me, therefore, to take the most drastic example I can find. Imagine that you are struck with a stick. What do you feel, the stick or its energy? The only possible answer is the energy, for the stick is the most harmless thing in the world, so long as it is not made to strike. But we can also run against a stick at rest. Very true; and what we feel then is, as I have already declared, the difference in the conditions of energy as opposed to our sense organs; and it is therefore all the same whether the stick is struck against us or we are pushed against
