A religion is a group or whole of religious phenomena - of religious beliefs, practices and institutions - so closely connected with one another as to be thereby differentiated from those of any other religion. Each religion has had a history, and its rise and spread, formation and transformations, as a religion, can only be truly traced by being historically traced. Also religions are historically connected, are related to one another, and have influenced one another, in ways which may be discovered, and can only be discovered, by historical research. Hence the History of Religions is also the history of religion, not an aggregation of the histories of particular religions, but a truly general history. Like the histories of art, industry, science, and society in general, it is found on examination to have been a process of development in which each stage of religion has proceeded gradually from antecedent factors and conditions. The precise nature of the development can only be ascertained by investigation of the history itself. No hypothesis of development should be assumed as a presupposition of such investigation. Naturalistic apriorism is as illegitimate in historical inquiry as theological or metaphysical apriorism.

The history of religion is not only of great importance in itself, but indispensable to the right understanding of general history, of the history of art, of philosophy, etc. It has been studied with more zeal and success during the nineteenth century than in all the preceding ages. The history of religious beliefs is, of course, only a part of the history of religions. If is, however, distinguishable, although inseparable from it, and is often and conveniently designated Comparative Theology. It comprehends comparative mythology and the history of doctrines, myths being beliefs which are mainly the products of imagination and doctrines of reflection.

The Psychology of Religion, the History of Religions, and Comparative Theology are clearly distinct, and ought not to be confounded. At the same time they are closely connected. They agree in that they are alike occupied with religion as an empirical fact. Hence they may be regarded as parts of a comprehensive science, to which it might be well to confine the designation "Science of Religions," instead of using it in the vague and ambiguous way which is so common. Thus understood, the Science of Religions may be said to deal with religion as a phenomenon of experience, whether outwardly manifested in history or inwardly realized in consciousness; to seek to describe and explain religious experience so far as it can be described and explained without transcending the religious experience itself. Its students have only to ascertain, analyse, explain and exhibit experienced fact. Were religion a physical fact, to study it merely as a fact would be enough. The astronomer, the naturalist, the chemist have no need to judge their facts; they have only to describe them, analyze them, and determine their relations. But it is otherwise with the students of religion, of morality, of art, of reasoning.

They soon come to a point where they must become judges of the phenomena and pronounce on their truth and worth. Experience in the physical sphere is experience and nothing more; experience in the spiritual sphere is very often experience of what is irreverent and impious, immoral and vicious, ugly and erroneous, foolish or insane. Has the mind simply to describe and analyze, accept and be content with such experience? Even the logician and the aesthetician will answer in the negative, will claim to judge their facts as conforming to or contravening the laws of truth and the ideals of art. Still more decidedly must the moralist and the student of religion so answer. Religion, then, is not completely studied when it is only studied historically. Hence it must be dealt with by other sciences or disciplines than those which are merely historical.

All the particular theological sciences or disciplines treat of particular aspects of religion or of religion in particular ways. Their relationships to one another can only be determined by their relationship to it. They can only be unified and co-ordinated in a truly organic manner by their due reference to it. When religion is studied not merely in particular aspects and ways, but in its unity and entirety, with a view to its comprehension in its essence and all essential relations, it is the object of the Philosophy of Religion. Although a distinct and essential department of philosophy, and the highest and most comprehensive theological science, the philosophy of religion could only have appeared in an independent and appropriate form when both philosophy and theology were highly developed.