It is a fact of the greatest significance that the words by which we convey our ideas of value always occur in pairs, one of which is the opposite of the other. In esthetics the beautiful is contrasted with the ugly, and the charming with the disgusting; in logic the true clashes with the false, and in philosophy the real with the unreal; in matters of public and private economy the cheap is antithetical to the expensive, and the generous to the miserly; while in ethics we constantly hear the words good and bad, right and wrong, virtuous and vicious employed to denote the opposition and contradiction of human interests and ideals. What is the ultimate reason why we thus employ such pairs of antonyms in our judgments of praise or blame, our expressions of desire or aversion, and in our estimations of merit or defect? This modern question, upon which the founding of a true science of ethics depends, cannot be answered by any appeal to the speculative metaphysics of bygone generations. We have passed out of that period in which men were content with an explanation based on the Zoroastrian hypothesis of a world divided between the warring forces of light and darkness, while our ears are now equally unresponsive to the bi-polar principles of love and hate which Empedocles propounded. Even our recent Emerson’s “Law of Compensation,” born though it was of canny, scientific thinking, is not marked by the least sign of that finality which the answer we seek should possess. It is, indeed, our conviction that the presence of antonyms in human speech is not to be accounted for by the assumption of a theory concerning the world as a whole, but rather by an examination of some of the baldest facts of our everyday experience. Nay more, it is our declared purpose to show that the metaphysical and theological notions of Empedocles and Zoroaster are themselves to be explained by reference to the physics and the physiology of man,—in terms of the structure and functions of the human body. But before we can comprehend the significance of such a thesis, it is first necessary to understand some of the physiological mechanisms underlying thought and speech. _The Rise of Mechanistic Psychology_