The contradistinction between force and matter is as old as that between idealism and materialism. The first conciliation between the two was attempted by imagination which, through the belief in spirits, suggested a secret nature as the cause of all natural phenomena. Science has of late expelled many of these special spirits by replacing the fantastic demons with scientific, or general, explanations. And after we have succeeded in explaining the demon of "pure" reason, it is not difficult to expel the special spirit of force by the general explanation of its nature and thus to reconcile scientifically the contradiction between spiritualism and materialism. In the universe which constitutes the object of science and of the faculty of reason, both force and matter are unseparated. In the world of sense perceptions force is matter and matter is force. "Force cannot be seen." Oh, yes! Seeing itself is pure force. Seeing is as much an effect of its object as an effect of the eye, and this double effect and other effects are forces. We do not see the things themselves, but their effects on our eyes. We see their forces. And force cannot alone be seen, it can also be heard, smelled, tasted, felt. Who will deny that he can feel the force of heat, of cold, of gravitation? We have already quoted the words of Professor Koppe to the effect that we "cannot perceive heat itself, we merely conclude from its effects that this force exists in nature." This is saying in other words that we do not see, hear, or feel the things themselves, but their effects or forces.