When this stage had been reached, the Reformation began to wear a new aspect, but several years elapsed before I saw whither my studies led. Only very slowly did a sequence of cause and effect take shape in my mind, a sequence wholly unexpected in character, whose growth resembled the arrangement of the fragments of an inscription, which cannot be read until the stones have been set in a determined order. Finally, as the historical work neared an end, I perceived that the intellectual phenomena under examination fell into a series which seemed to correspond, somewhat closely, with the laws which are supposed to regulate the movements of the material universe. Theories can be tested only by applying them to facts, and the facts relating to successive phases of human thought, whether conscious or unconscious, constitute history; therefore, if intellectual phenomena are evolved in a regular sequence, history, like matter, must be governed by law. In support of such a conjecture, I venture to offer an hypothesis by which to classify a few of the more interesting intellectual phases through which human society must, apparently, pass, in its oscillations between barbarism and civilization, or, what amounts to the same thing, in its movement from a condition of physical dispersion to one of concentration. The accompanying volume contains the evidence which suggested the hypothesis, although, it seems hardly necessary to add, an essay of this size on so vast a subject can only be regarded as a suggestion. The theory proposed is based upon the accepted scientific principle that the law of force and energy is of universal application in nature, and that animal life is one of the outlets through which solar energy is dissipated. Starting from this fundamental proposition, the first deduction is, that, as human societies are forms of animal life, these societies must differ among themselves in energy, in proportion as nature has endowed them, more or less abundantly, with energetic material. Thought is one of the manifestations of human energy, and among the earlier and simpler phases of thought, two stand conspicuous--Fear and Greed. Fear, which, by stimulating the imagination, creates a belief in an invisible world, and ultimately develops a priesthood; and Greed, which dissipates energy in war and trade. Probably the velocity of the social movement of any community is proportionate to its energy and mass, and its centralization is proportionate to its velocity; therefore, as human movement is accelerated, societies centralize. In the earlier stages of concentration, fear appears to be the channel through which energy finds the readiest outlet; accordingly, in primitive and scattered communities, the imagination is vivid, and the mental types produced are religious, military, artistic. As consolidation advances, fear yields to greed, and the economic organism tends to supersede the emotional and martial.