Finally, the reservations made in the case of works dealing with the Philosophy of history are to be repeated for those dealing with the Philosophy of nature. In them, too, there is something more than, and something different from, the sterile analogical exercises that we have mentioned. Some of the philosophers of nature, in the pursuit of their illusions, have made occasional scientific discoveries, in the same way that the alchemists seeking the philosopher's stone made discoveries in Chemistry. Those discoveries in physical and natural science cannot serve to increase the value of the theory of the Philosophy of nature any more than those made in chemistry increased the value of alchemy. But they confer value on the books entitled Philosophy of nature, and do honour to their authors as physicists, not as metaphysicians. From the philosophical point of view, those works have had the merit of affirming, though but in imaginative and symbolical ways, the unity and spirituality of nature, opening the path to its unification with the history of man. They have the yet greater merit of contributing effectively in the battle engaged by them against the sciences of making clear the empirical character of the naturalistic concepts and the abstract character of the mathematical. Nevertheless, they drew illegitimate conclusions from such gnoseological truth and carried on a war of conquest, which must be held to be unjust. In virtue of the positive elements that they contain, works on the Philosophy of nature have aided the advance both of the sciences and of philosophy, which in their properly philosophico-naturalistic parts they have violated and debased and forced into hybrid unions. [Sidenote: _Contemporary demands for a Philosophy of nature and their various meanings._]