But since the human mind itself is the natural instrument whereby man acquires _all_ his knowledge, it will be at once apparent that the study of the phenomenon of _knowledge_ itself, of the _cognitive_ activity of the mind, can be studied, and must be studied, not merely as a natural phenomenon of the mind, but from the point of view of _its special significance as representative_ of objects other than itself, from the point of view of _its validity or invalidity_, _its truth or falsity_, and with the special aim of determining the scope and limitations and conditions of its objective validity. We have already referred to the study of human knowledge from this standpoint, in connexion with what was said above concerning Logic. It has a close kinship with Logic on the one hand, and with Psychology on the other; and nowadays it forms a distinct branch of speculative Philosophy under the title of _Criteriology_, _Epistemology_, or the _Theory of Knowledge_. Arising out of the data of our direct experience, external and internal, as studied in the philosophical departments just outlined, we find a variety of evidences all pointing beyond the domain of this direct experience to the supreme conclusion that there exists of necessity, distinct from this directly experienced universe, as its Creator, Conserver, and Ruler, its First Beginning and its Last End, its _Alpha_ and _Omega_, One Divine and Infinite Being, the Deity. The existence and attributes of the Deity, and the relations of man and the universe to the Deity, form the subject-matter of _Natural Theology_. VII. DEPARTMENTS OF METAPHYSICS: ONTOLOGY AND EPISTEMOLOGY.—According to the Aristotelian and scholastic conception speculative philosophy would utilize as data the conclusions of the special sciences—physical, biological, and human. It would try to reach a deeper explanation of their data by synthesizing these under the wider aspects of change, quantity, and being, thus bringing to light the ultimate causes, reasons, and explanatory principles of things. This whole study would naturally fall into two great branches: General Metaphysics (_Cosmology_ and _Psychology_), which would study things exempt from quantity and change not really but only by mental abstraction; and Special Metaphysics (_Natural Theology_), which would study the positively immaterial and immutable Being of the Deity.