No one now doubts that animals feel, hear, remember, and the like, while man is able to exercise his will, to feel, to remember, deliberately to consider all his actions and functions, because he not only possesses the direct and spontaneous intuition with respect to himself and things in general which he has in common with animals, but he has an intuitive knowledge of that intuition itself, and in this way he multiplies within himself the exercise of his whole psychical life. We find the ultimate cause of this return upon himself, and his intuition of things, in his deliberate will, which does not only immediately command his body and his manifold relative functions, but also the complex range of his psychical acts. This fact, which as I believe has not been observed before, is of great importance. It is manifest that the difference between man and other animals does not consist in the diversity or discrepancy of the elements of the intelligence, but in its reflex action on itself; an action which certainly has its conditions fixed by the organic and physiological composition of the brain. If it should be said that the traditional opinion of science, as well as the general sentence of mankind, have always regarded reflection as the basis of the difference between animals and man, so that there is no novelty in our principle, the assertion is erroneous. Reflection, as an inward psychical fact, has certainly been observed by psychologists and philosophers in all civilized times, and instinctively by every one; nor could it be otherwise, since reflection is one of the facts most evident to human consciousness. But although the fact, or the intrinsic and characteristic action of human thought has been observed, and has often been discussed and analyzed in some of its elements, yet its genesis has not been declared, nor has its ultimate cause been discovered. We propose to discover this ultimate cause, and we refer it to the exercise of the will over all the elements and acts which constitute human intelligence; an intelligence only differing from that of animals by this inward and deliberate fact, which enables man to consider and examine all his acts, thus logically doubling their range. This intelligence has in animals a simple and direct influence on their bodies and on the external world, in proportion to their diverse forms and inherited instincts; while in man, owing to his commanding attitude, it falls back upon itself, and gives rise to the inquiring and reflective habit of science.