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certainty of the knowledge both of the internal and the external world that accompanies perception. We will rapidly review each of these points in their theoretical and historical bearings.

The physical conditions of sight, hearing, touch, taste, and smell are matters purely of physical inquiry. A satisfactory knowledge of the intervening means of perception is comparatively recent, as is all exact anatomical science. An inquiry into the physical incidents of perception has served to displace some crude theories of its method, and to make way for a more careful separation of the material and intellectual elements in the process, or, in the minds of some, for their more complete identification. The notion of "images," "species," "representative ideas," which mediate between the object and the mind, arose out of the ignorance of the organs of sense, and lingered in philosophy as late as the time of Locke. These images or ideas had one or another degree of materiality according to the age, the philosophy, the person, who dealt with them. Earlier, they were a physical film passing off from the object; later, as the immateriality of the mind gained ground, they shared more of its nature; but at no time could they perform their office. To whichever extreme they moved, they thereby lost power to touch the other. If spiritual, they were out of relation with matter, if material, they were disassociated with mind. A knowledge of the eye, the ear, the brain sweeps away this intermediate mechanism, establishes the complete physical character of the organic portion of perception, the complete spiritual nature of the mental portion, and leaves the interaction of the two an ultimate fact.

Descartes sharply distinguished between mind and matter, and so relegated physical inquiry to its own physical field, and mental inquiry to consciousness. All media