If we have got this definition of the two forms of energy clearly into our heads, we shall be the better prepared for this further generalisation—the grandest, perhaps, in the whole range of modern science—that energy, like matter, is indestructible, and can only be transformed, but never created or annihilated. This is at first sight a more difficult proposition to establish in the case of energy than in that of matter. In the latter case we have nothing in our experience that can lead us to suppose that we have ever created something out of nothing; but in the former, our first impression undoubtedly is that we do create force. If I throw a stone at a bird I have an instinctive impression that the force which projects the stone is the creation of my own conscious will; that I had the choice either to throw or not to throw; and that if I had decided not to throw, the impelling force would never have existed. But, if we look more closely at the matter, it is not really so. The chain of events is this: the first impulse proceeds from the visual rays, which, concentrated by the lens of the eye on the retina, give an image of the bird; this sends vibrations along the optic nerve to the brain, setting in motion certain molecules of that organ; these again send vibrations along other nerves to certain muscles of the arm and hand, which contract, and by doing so give out the energy of movement which throws the stone. All this process is strictly mechanical; the eye acts precisely like a camera obscura in forming the image; the nerve-vibrations, though not identical with those of the wires of an electric telegraph, are of the same nature, their velocity can be measured, and their presence detected by the galvanometer; the energy of the muscle is stored there by the slow combustion of the food we have eaten, in the oxygen of the air we have breathed. Take any of these conditions away, and no effort of the will can produce the result. If the nerve is paralysed, or the muscle, from prolonged starvation, has no energy left, the stone will not be thrown, however much we may desire to kill the bird.