In the fourth place, as the force of the will is both higher and better known than the mechanical, chemical, and vital forces of nature, we are warranted in interpreting the lower by the higher, and not in reducing the higher to the level of the lower. As we ascend in nature from the lowest vital forms to the highest type of organization, we find that the higher is not only an advance upon the lower, but that it _includes_ it; and no naturalist would describe a vertebrated animal by that which it held in common with the mollusca. That in which it differs from the types beneath it is held to be its distinctive and descriptive feature. When, therefore, we reach man at the top of the scale, separated by a distinct endowment from the classes beneath him, yet conserving all their main characteristics in his nature, and describe him not by what he has in common with the lower animals, but by that in which he differs from them, we act on the principle of selecting the highest feature we can find, and taking it as our guide. And similarly when we are in search of the Supreme Principle of the universe, the _causa causarum_, we interpret it by the highest features in human nature, because that nature is the highest with which we are experimentally acquainted. And we may validly throw the burden of proof upon the positivist, and ask why the great cosmical force that rules in nature should be radically different from the volitional force which is the root of our personality? Reverting again to the force of gravitation, why should it not be the outcome in nature of a Will vaster than man's, resembling, yet transcending it? To what does that force amount? The phenomenalist cannot arrest our inquiry by simply drawing the veil of nescience over it. He cannot slip a lid over the end of our telescope turned skyward by merely exclaiming 'mystery of mysteries, all is mystery.' And it seems to us that we must either divest the word gravitation of all intelligible meaning, or while perceiving the unlikeness at a glance, we must 'invest it with a human or _quasi-human_ vitality.'