In addition there is still another class of philosophers, those who dispute the possibility of a perception of the universe or at least of an exhaustive perception. To them belong, among the moderns, Hume and Kant, and they have played a very distinguished role in the evolution of philosophy. This point of view has been now refuted by Hegel, as far as possible, from the idealistic standpoint. The materialistic additions made by Feuerbach are more ingenious than deep. The most destructive refutation of this as of all other fixed philosophic ideas is actual result, namely experiment and industry. If we can prove the correctness of our idea of an actual occurrence by experiencing it ourselves and producing it from its constituent elements, and using it for our own purposes into the bargain, the Kantian phrase "Ding an Sich" (thing in itself) ceases to have any meaning. The chemical substances which go to form the bodies of plants and animals remained just such "Dinge an Sich" until organic chemistry undertook to show them one after the other, whereupon the thing in itself became a thing for us, as the coloring matter in the roots of madder, alizarin, which we no longer allow to grow in the roots of the madder in the field, but make much more cheaply and simply from coal tar. The Copernican system was for three hundred years a hypothesis, with a hundred, a thousand, or ten thousand chances in its favor, but still a hypothesis. But when Leverrier by means of the data of this system not only discovered the existence of a certain unknown planet, but even calculated the position in the heavens which this planet must necessarily occupy, and when Galles really found this planet, then the Copernican system was proved. If, nevertheless, the resurrection of the Kantian idea in Germany is being tried by the Neo-Kantians, and of that of Hume in England (where they never died), by the agnostics, that is, in the face of the long past theoretical and practical refutation of these doctrines, scientifically, a step backwards, and practically, merely the acceptance of materialism in a shame-faced way, clandestinely, and the denial of it before the world. But the philosophers were during this long period from Descartes to Hegel and from Hobbes to Feuerbach by no means, as they thought, impelled solely by the force of pure reason. On the contrary, what really impelled them was, in particular, the strong and ever quicker conquering step of natural science and industry. Among the materialists this very quickly showed itself on the surface, but the idealistic systems filled themselves more and more with materialistic content and sought to reconcile the antagonism between spirit and matter by means of pantheism, so that finally the Hegelian system represented merely a materialism turned upside down, according to idealistic method and content.