True, there is perhaps matter—for instance, the ether—which we neither see nor feel, but which still exists. This matter is then qualified by some other form of energy by which it manifests itself. Thus we comprehend ether as light, heat and colors, all forces, as well as gravity, electricity, etc. Already from these suggestions it is evident that force is the only substantial thing in the material world. Without force, matter is nothing that may be comprehended either by the senses or by the reason. What we call matter is nothing but different kinds of energy.[1] We have space-occupying energy, chemical, electrical, mechanical forms of energy, and so forth. How are these forms of energy related to each other? Between forms so different as tones and light, colors and mechanical work, there is at least no connection apparent to external observation. For a long time it was also believed that no such relation existed. It was only after 1840 that several scientists made the startling discovery almost simultaneously that physical forces may be transformed one into another. It proved possible to transform a certain quantity of heat into an equal quantity of mechanical energy, which again might be turned into equivalent quantities of electricity, light, chemical energy, etc. It was further found that these processes might be undertaken in the reverse order, so that the original form of energy could be restored in unchanged quantity and with unmodified qualities. Nothing was lost and nothing was added. Recent science is founded entirely on these facts, which later generations probably will consider as the greatest of all the discoveries of the last century. This law of the permanence and the mutability of force is of immediate importance to materialism. As long as it was thought that the forces of nature were separate and different from each other, it was easy to imagine that the more inaccessible or mystic forms stood nearer life, yea, were life itself. The absurdity of such an idea is now obvious, since it has been shown that the physical forces may be transformed into one another and therefore are not intrinsically separate, but fundamentally the same force, acting differently under different conditions. Now, if life were a form of material energy, any form of physical force might be transformed into life and consciousness, into spiritual and moral forces. Life and consciousness might then be artificially produced, and we would rack our brains in order to find the mechanical equivalent of the intellect, try to measure it in amperes and volts, etc. But nothing of this kind is done, simply because it is impossible, as presently we shall see. _Life cannot be transformed into any form of material energy, and, vice versa, no form of material energy can be transformed into life._ Life and physical force are, as to nature and substance, essentially different principles.