The answer to this final question naturally varies considerably according to the stand-point of the philosophic inquirer and his empirical acquaintance with the real world. We grant at once that the innermost character of nature is just as little understood by us as it was by Anaximander and Empedocles twenty-four hundred years ago, by Spinoza and Newton two hundred years ago, and by Kant and Goethe one hundred years ago. We must even grant that this essence of substance becomes more mysterious and enigmatic the deeper we penetrate into the knowledge of its attributes, matter and energy, and the more thoroughly we study its countless phenomenal forms and their evolution. We do not know the "thing in itself" that lies behind these knowable phenomena. But why trouble about this enigmatic "thing in itself" when we have no means of investigating it, when we do not even clearly know whether it exists or not? Let us, then, leave the fruitless brooding over this ideal phantom to the "pure metaphysician," and let us instead, as "real physicists," rejoice in the immense progress which has been actually made by our monistic philosophy of nature. Towering above all the achievements and discoveries of the century we have the great, comprehensive "law of substance," the fundamental law of the constancy of matter and force. The fact that substance is everywhere subject to eternal movement and transformation gives it the character also of the universal law of evolution. As this supreme law has been firmly established, and all others are subordinate to it, we arrive at a conviction of the universal unity of nature and the eternal validity of its laws. From the gloomy _problem_ of substance we have evolved the clear _law_ of substance. The monism of the cosmos which we establish thereon proclaims the absolute dominion of "the great eternal iron laws" throughout the universe. It thus shatters, at the same time, the three central dogmas of the dualistic philosophy--the personality of God, the immortality of the soul, and the freedom of the will. Many of us certainly view with sharp regret, or even with a profound sorrow, the death of the gods that were so much to our parents and ancestors. We must console ourselves in the words of the poet: "The times are changed, old systems fall, And new life o'er their ruins dawns." The older view of idealistic dualism is breaking up with all its mystic and anthropistic dogmas; but upon the vast field of ruins rises, majestic and brilliant, the new sun of our realistic monism, which reveals to us the wonderful temple of nature in all its beauty. In the sincere cult of "the true, the good, and the beautiful," which is the heart of our new monistic religion, we find ample compensation for the anthropistic ideals of "God, freedom, and immortality" which we have lost.