The natural sciences as empirical concepts, and their practical nature--Elimination of an equivocation concerning this practical character--Impossibility of unifying them in one concept--Impossibility of introducing into them rigorous divisions--Laws in the natural sciences, and so-called prevision--Empirical character of naturalistic laws--The postulate of the uniformity of nature, and its meaning--Pretended impossibility of exceptions to natural laws--Nature and its various meanings. Nature as passivity and negativity--Nature as practical activity--Nature in its gnoseological significance, as naturalistic or empirical method--The illusions of materialists and dualists--Nature as empirical distinction of an inferior reality in respect to a superior reality--The naturalistic method, and the natural sciences as extending to superior not less than to inferior reality--Claim for such extension, and effective existence of what is claimed--Historical foundation of the natural sciences--The question whether history be foundation or crown of thought--Naturalists as historical investigators--Prejudices as to non-historicity of nature--Philosophic foundation of the natural sciences, and effect of philosophy upon them--Effect of natural sciences upon philosophy, and errors in conceiving such relation--Reason of these errors. Naturalistic philosophy--Philosophy as the destroyer of naturalistic philosophy, but not of the natural sciences. Autonomy of these. VI MATHEMATICS AND THE MATHEMATICAL SCIENCE OF NATURE Idea of a mathematical science of nature--Various definitions of mathematics--Mathematical procedure--Apriority of mathematical principles--Contradictoriness of the _a priori_ principles. They are not thinkable, and not intuitive--Identification of mathematics with abstract pseudoconcepts--The ultimate end of mathematics: to enumerate, and, therefore, to aid the determination of the single. Its place--Particular questions concerning mathematics--Rigour of mathematics and rigour of philosophy--Loves and hates between the two forms--Impossibility of reducing the empirical sciences to the mathematical; and the empirical limits of the mathematical science of nature--Decreasing utility of mathematics in the loftiest spheres of the real. VII THE CLASSIFICATION OF THE SCIENCES Theory of the forms of knowledge and doctrine of the categories--Problem of classification of the sciences; its empirical nature--Falsely philosophic character that it assumes--Coincidence of that problem with the search for the categories, when understood with philosophic rigour--Forms of knowledge and literary-didactic forms--Prejudices derived from the latter--Methodical prologues to scholastic manuals, their impotence--Capricious multiplication of the sciences--The sciences and professional prejudices. THIRD PART THE FORMS OF ERROR AND THE SEARCH FOR TRUTH I ERROR AND ITS NECESSARY FORMS