When philosophy, as a search for truth in the highest sense, thus unites our isolated discoveries and seeks to weld them into one unified system of the world, it comes at length to state certain fundamental problems, the answer to which varies according to the degree of culture and the point of view of the inquirer. These final and highest objects of scientific inquiry have been of late comprehended under the title of _The Riddle of the Universe_, and I gave this name to the work I published in 1899, which dealt with them, in order to make its aim perfectly clear. In the first chapter I dealt briefly with what have been called "the seven great cosmic problems," and in the twelfth chapter I endeavored to show that they may all be reduced to one final "problem of substance," or one great "riddle of the universe." The general formulation of this problem is effected by blending the two chief cosmic laws--the chemical law of the constancy of matter (Lavoisier, 1789), and the physical law of the constancy of force (Robert Mayer, 1842). This monistic association of the two fundamental laws, and establishment of the unified law of substance, has met with a good deal of agreement, but also with some opposition; but the most violent attacks were directed against my monistic theory of knowledge, or against the method I followed in seeking to solve the riddle of the universe. The only paths which I had recognized as profitable were those of experience and thought--or empirical knowledge and speculation. I had insisted that these two methods supplemented each other, and that they alone, under the direction of reason, lead to the attainment of truth. At the same time I had rejected as false two other much-frequented paths which purported to lead directly to a profounder knowledge, the ways of emotion and revelation; both of these are in opposition to reason, since they demand a belief in miracles.