The two original characteristics of M. Sixte’s inquiries are found elsewhere. The first resides in a negative analysis of what Herbert Spencer calls the Unknowable. We know that the great English thinker admits that all reality rests upon a First Cause which it is impossible to penetrate; consequently it is necessary to use the formula of Fichte, to comprehend this First Cause (_arrière-fonds_) as incomprehensible; but as the beginning of the “First Principles” strongly attests this Unknowable is real to Mr. Spencer. It exists since we derive our existence from it. From this there is only a step to apprehend that this First Cause of all reality involves a mind and a soul since one finds their source in it. Many excellent minds foresee a probable reconciliation between science and religion on this ground of the Unknowable. For M. Sixte this is a last form of metaphysical illusion which he is rabid to destroy with an energy of argument that has not been seen to this degree since Kant. His second title of honor as psychologist consists in an exposé, quite novel and very ingenious, of the animal origin of human sensibility. Thanks to an exhaustive reading and a minute knowledge of the natural sciences, he has been able to attempt for the genesis of human thought the work which Darwin attempted for the genesis of the forms of life. Applying the law of evolution to all the facts which constitute the human heart, he has claimed to show that, our most refined sensations, our most subtile moral delicacies as well as our most shameful failures, are the latest development, the supreme metamorphosis of very simple instincts, which are themselves transformations of the primitive cellule; so that the moral universe exactly reproduces the physical, and the former is the consciousness, either painful or pleasurable, of the latter. This conclusion presented under the title of hypothesis because of its metaphysical character, is the result of a marvelous series of analyses, among which it is proper to cite two hundred pages on love, which are so audacious as to be almost ludicrous from the pen of so chaste a man. But has not Spinoza himself given us a theory of jealousy which has not been equalled in brutality by any modern novelist? And does not Schopenhauer rival Chamfort in the spirit of his tirades against women?