At the close of our philosophic study of the riddles of the universe we turn with confidence to the answer to the momentous question, How nearly have we approached to a solution of them? What is the value of the immense progress which the passing nineteenth century has made in the knowledge of nature? And what prospect does it open out to us for the future, for the further development of our system in the twentieth century, at the threshold of which we pause? Every unprejudiced thinker who impartially considers the solid progress of our empirical science, and the unity and clearness of our philosophic interpretation of it, will share our view: the nineteenth century has made greater progress in knowledge of the world and in grasp of its nature than all its predecessors; it has solved many great problems that seemed insoluble a hundred years ago; it has opened out to us new provinces of learning, the very existence of which was unsuspected at the beginning of the century.