[Footnote 35: New Plane and Solid Geometry. By W. W. Beman and D. E. Smith. Boston: Ginn & Co. Pp. 382.] The plan of the investigation undertaken by Mr. _Walter Smith_ in his _Methods of Knowledge_[36] is, first, to give a definition of knowledge. The methods are then considered by which men have thought it possible to attain knowledge of the self on the one hand, and the not-self on the other. The common view of philosophers and men of science that truth is given in general concepts, or universals, or categories, is taken up, and the special form of the doctrine given in empiricism is considered and found to be a doctrine wanting in all its forms. Yet it is pointed out that the concept has its uses in the mental economy. The method is then expounded of knowing the not-self as being gained by sympathetic imitation. It is then determined wherein self-knowledge consists, and the bearing of this theory on the philosophical problem and on certain practical questions is indicated. [Footnote 36: Methods of Knowledge. An Essay in Epistemology. New York: The Macmillan Company. Pp. 340. Price, $1.25.] In _The Philosophy of Memory and Other Essays_[37] Dr. _D. T. Smith_ develops a theory of mental action, the basis of which is the setting up in the cells of the gray matter of the brain, and possibly of the spinal cord, of orderly grouping of waves or vibrations among certain atoms or molecules by whatever may affect any of the senses; that these undulations are realized first as sensations, and then group themselves so as to form perceptions, ideas, emotions, etc. They rise in succession into the scope of consciousness. After a time the effect of these vibrations in consciousness is weakened, without perhaps utterly passing away, and retains the possibility of being re-enforced by kindred vibrations in harmony with it. This is memory. [Footnote 37: The Philosophy of Memory and Other Essays. By D. T. Smith. Louisville, Ky.: John P. Morton & Co. Pp. 203.]