Descartes, one must bear in mind, divided the universe into two great realms of being, namely: the conscious and the unconscious, the _psychic_ world of mind and the _physical_ world of matter, unextended substance which thinks and extended substance which moves. In man these two substantial principles were conceived as being united by the tenuous link of mere contact, the spirit or mind remaining separate from, and unmingled with, its material partner, the body. The main trouble with this dualism is that it draws the line of demarcation at the wrong place. Reason and sense-consciousness are bracketed together above the line as being equally spiritual; physiological processes and processes purely physicochemical are coupled below the line as being equally mechanical. Now, when a brain-function such as sense-perception is introduced, like another Trojan Horse, into the citadel of spiritualism, it is a comparatively easy task for materialism to storm and sack that citadel by demonstrating with a thousand neuro-physiological facts that all sensory functions are rigidly correlated with neurological processes, that they are, in short, functions of the nervous system, and therefore purely material in nature. On the other hand, once we retreat from the trench of distinction between the processes of unconscious or vegetative life and the physicochemical processes of the inorganic world, that moment we have lost the strategic position in the conflict with mechanism, and nothing avails to stay its triumphant onrush. Hence, from first to last, it is perfectly clear that the treacherous psychophysical dualism of Descartes has done far more harm to the cause of spiritualism than all the assaults of materialism. There is a Latin maxim which says: _Extrema sese tangunt_—“Extremes come in contact with each other.” The ultraspiritualism of Descartes by confounding spiritual, with organic consciousness, leads by the most direct route to the opposite extreme of crass materialism.