From a time beyond the dawn of history, humanity has been seeking to explain the universe. At first, the effort did not concern itself further than to make a supposition as to the causes of the various phenomena presented to the senses. As knowledge increased, first by observation and later by experiment also, the ideas as to these causes passed progressively through three stages—the theological (the causes were thought to be spirits or gods); the metaphysical (the causes were thought in this secondary or intermediate stage to be some inherent, animating, energizing principles); and the scientific (the causes were finally thought of as simply mechanical, chemical, and magneto-electrical attractions and repulsions, qualities or characteristics of matter itself, or of the thing of which matter is itself composed.)

With an increase of knowledge and the inquiry as to the nature of causes, there arose an inquiry into reality. What was the essential nature of the stuff of which the universe was made, what was the matter, what were things in themselves, what were the noumena (the realities), lying back of the phenomena (the appearances)? Gradually ideas explaining motion, force, and energy were developed. At the same time, an inquiry was made as to the nature of man, the working of his mind, the nature of thought, the relation of his concepts (ideas) to his perceptions (knowledge gained through the sense), and the relations of both to the noumena (realities).