"All natural science is philosophy, and all true philosophy is natural science. All true science is natural philosophy." I expressed in these words the general result of my monistic studies in 1866 (in the twenty-seventh chapter of my _Generelle Morphologie_). I then laid it down as the fundamental principle of the monistic system that the unity of nature and the unity of science follow absolutely from any connected study of modern philosophic science, and I expressed my conviction in these terms: "All human science is knowledge based on experience, or empirical philosophy; or, if the title be preferred, philosophic empiricism. Thoughtful experience, or thought based on experience, is the only way and method to be followed in the search for truth." I endeavored to establish these theses conclusively in the first book of the _Generelle Morphologie_, which contains (p. 108) a critical and methodological introduction to this science. Not only are those methods considered "which must necessarily supplement each other" (I. Empiricism and Philosophy; II. Analysis and Synthesis; III. Induction and Deduction), but also those "which necessarily exclude each other" (IV. Dogmatism and Criticism; V. Teleology and Causality, or Vitalism and Mechanicism; VI. Dualism and Monism). The monistic principles which I developed there thirty-eight years ago have only been confirmed by my subsequent labors, and so I may refer the interested reader to that work. _The Riddle of the Universe_ is in the main an attempt to introduce to the general reader in a convenient form the chief points of the monistic system I established. However, the opposition which has been aroused by the general philosophic observations of the _Riddle_ compels me to give a further explanation of the chief features of my theory of knowledge.