For thousands of years humanity has placed the last and supreme basis of all phenomena in an efficient cause, to which it gives the title of God (deus, theos). Like all general ideas, this notion of God has undergone a series of remarkable modifications and transformations in the course of the evolution of reason. Indeed, it may be said that no other idea has had so many metamorphoses; for no other belief affects in so high a degree the chief objects of the mind and of rational science, as well as the deepest interests of the emotion and poetic fancy of the believer.

A comparative criticism of the many different forms of the idea of God would be extremely interesting and instructive; but we have not space for it in the present work. We must be content with a passing glance at the most important forms of the belief and their relation to the modern thought that has been evoked by a sound study of nature.