This unnatural and fatal opposition between science and philosophy, between the results of experience and of thought, is undoubtedly becoming more and more onerous and painful to thoughtful people. That is easily proved by the increasing spread of the immense popular literature of "natural philosophy" which has sprung up in the course of the last half-century. It is seen, too, in the welcome fact that, in spite of the mutual aversion of the scientific observer and the speculative philosopher, nevertheless eminent thinkers from both camps league themselves in a united effort to attain the solution of that highest object of inquiry which we briefly denominate the "world-riddles." The studies of these "world-riddles" which I offer in the present work cannot reasonably claim to give a perfect solution of them; they merely offer to a wide circle of readers a critical inquiry into the problem, and seek to answer the question as to how nearly we have approached that solution at the present day. What stage in the attainment of truth have we actually arrived at in this closing year of the nineteenth century? What progress have we really made during its course towards that immeasurably distant goal? The answer which I give to these great questions must, naturally, be merely subjective and only partly correct; for my knowledge of nature and my ability to interpret its objective reality are limited, as are those of every man. The one point that I can claim for it, and which, indeed, I must ask of my strongest opponents, is that my Monistic Philosophy is sincere from beginning to end--it is the complete expression of the conviction that has come to me, after many years of ardent research into Nature and unceasing reflection, as to the true basis of its phenomena. For fully half a century has my mind's work proceeded, and I now, in my sixty-sixth year, may venture to claim that it is mature; I am fully convinced that this "ripe fruit" of the tree of knowledge will receive no important addition and suffer no substantial modification during the brief spell of life that remains to me.