=The Fundamental Principle in the Teaching of Hobbes.= The assumption from which Hobbes deduced his entire philosophy was the mechanical conception of the physical world,――the characteristic philosophical assumption of his age. Hobbes’s contemporaries, both the natural scientists and the philosophers, had, however, on the whole, restricted the conception of mechanism to the physical world. Hobbes differed from them all in universalizing the conception. He extended its application from the physical over upon the mental realm, and thereby reduced the mental world to physics. He stated this mechanical principle in two parts: _all that exists is body_; _all that occurs is motion_. Hobbes applies this assumption to the physical world and it gives him materialism;[20] he applies it to knowledge and it gives him sensationalism;[21] he applies it to the will and it gives him determinism;[22] he applies it to morals and politics and it gives him naturalism.[23] Body is nature; body is everything. Body is the first term leading through man up to the State. With Hobbes, as with others of his time, the political field was the whole ground to be penetrated. The fundamental principle, by which Hobbes thought the whole field was to be explained, is body in motion. The mental world became drawn into the physical, and thereby his mechanical conception became the more natural. There was one realm which Hobbes left untouched by his principle: the realm of the spirit, _i. e._ God, souls, angels. The science of bodies cannot deal with the supernatural, for the supernatural does not consist of bodies in motion. Matter and mind are homogeneous; matter and spirit are not. The contrast in Hobbes is not between matter and mind, the material and the psychical, but between matter and spirit, the material and the supra-material.