The theories of the nature of matter and of percepts which we have been last discussing still maintain a precarious existence in Mr. Russell's recent works. When, however, the ground of inference with respect to the reality and nature of physical objects or events is thus altered, the content of that inference must be correspondingly transformed, or rather, attenuated. If perception occurs at the end of a long causal process extending from the external object to the brain of the percipient, it appears evident that the percept must exist at the place, as well as at the time, at which the process terminates and the percipient event occurs. The certainty of people's knowledge of the existence and character of the immediate datum seems to Mr. Russell to imply the intra-cranial situation of what is known. The only terms, in short, upon which the proposition that percepts are in people's brains can be accepted imply a sort of intra-cranial dualism.