§ 61. It must not be supposed that metaphysics is dogmatically committed to the reduction of all reality to a unity of nature. It is quite consistent with its purpose that the parts of reality should be found to compose a group, or an indefinite multitude of irreducibly different entities. But it is clear that even such an account of things deals with what is true of all reality, and even in acknowledging the variety of its constituents, attributes to them some kind of relationship. The degree to which such a relationship is regarded as intimate and essential, determines the degree to which any metaphysical system is _monistic_,[159:10] rather than _pluralistic_. But the significance of this difference will be better appreciated after a further differentiation of the metaphysical problem has been noted. [Sidenote: Ontology and Cosmology Concern Being and Process.] § 62. It has already been suggested that the test of Thales's conception lay in the possibility of deriving nature from it. A world principle must be fruitful. Now an abstract distinction has prevailed more or less persistently in metaphysics, between _the general definition of being_, called _ontology_, and the study of the processes wherewith being is divided into things and events. This latter study has to do primarily with the details of experience enumerated and systematized by the natural sciences. _To reconcile_ these, _or the course of nature, with the fundamental definition of being_, is the problem of _cosmology_. Cosmology is the construing of the _prima facie_ reality in terms of the essential reality. It is the proof and the explanation of ontology. Since the most familiar part of the _prima facie_ reality, the part almost exclusively noticed by the naive mind, is embraced within the field of the physical sciences, the term cosmology has come more definitely to signify the _philosophy of nature_. It embraces such an examination of space, time, matter, causality, etc., as seeks to answer the most general questions about them, and provide for them in the world thought of as most profoundly real. Such a study receives its philosophical character from its affiliation with ontology, as the latter would find its application in cosmology. [Sidenote: Mechanical and Teleological Cosmologies.]