The concept of hemeostasis (the principle of stability in physiological systems) originated by Cannon after many years of fruitful research, is applied by this author to the psychological realm. Spinoza, Smuts, and Whitehead had noticed this phenomenon in psychology and Fechner was given credit by Freud for emphasizing it. The author states that the psychology of color vision or of the after-image of motion, as well as certain aspects of nystagmus, are evidences of the existence of such a tendency toward psychological equilibrium. Fletcher adds 'likewise most if not all of the so-called defense mechanisms which represent reactions of human personality under conditions of disturbed status can be better understood if referred to a single, general explanatory principle than if merely described as symptoms or set up as isolated principles in themselves.'