Without in any way countenancing such an isolation of metaphysics from the positive sciences, we may, nevertheless, adopt the modern division in substance and in practice. While recognizing the intimate connexion between the special sciences and metaphysics in all its branches, we may regard as _General Metaphysics_ all inquiries into the fundamental principles of _being_ and of _knowing_, of _reality_ and of _knowledge_; and as _Special Metaphysics_ the philosophical study of physical nature, of human nature, and of God, the Author and Supreme Cause of all finite reality. Thus, while special metaphysics would embrace Cosmology, Psychology, and Natural Theology, general metaphysics would embrace Ontology and Epistemology. These two latter disciplines must no doubt investigate what is in a certain sense one and the same subject-matter, inasmuch as _knowledge_ is knowledge of reality, nor can the _knowing mind_ (the _subjectum cognoscens_) and the known reality (the _objectum cognitum_) be wholly separated or studied in complete isolation from each other. Yet the whole content of human experience, which forms their common subject-matter, can be regarded by mental abstraction from the two distinct standpoints of the knowing mind and the known reality, and can thus give rise to two distinct sets of problems. Epistemology is thus concerned with the truth and certitude of human knowledge; with the subjective conditions and the scope and limits of its validity; with the subjective or mental factors involved in knowing.(28) Ontology is concerned with the objects of knowledge, with reality considered in the widest, deepest, and most fundamental aspects under which it is conceived by the human mind: with the being and becoming of reality, its possibility and its actuality, its essence and its existence, its unity and plurality; with the aspects of truth, goodness, perfection, beauty, which it assumes in relation with our minds; with the contingency of finite reality and the grounds and implications both of its actual existence and of its intelligibility; with the modes of its concrete existence and behaviour, the supreme categories of reality as they are called: substance, individual nature, and personality; quantity, space and time, quality and relation, causality and purpose. These are the principal topics investigated in the present volume. The investigation is confined to fundamental concepts and principles, leaving their applications to be followed out in special metaphysics. Furthermore, the theory of knowledge known as _Moderate Realism_,(29) the Realism of Aristotle and the Scholastics, in regard to the validity of knowledge both sensual and intellectual, is assumed throughout: because not alone is this the true theory, but—as a natural consequence—it is the only theory which renders the individual things and events of human experience really intelligible, and at the same time keeps the highest and most abstract intellectual