§ 1. In our two remaining Books we shall have to deal with the more elementary of the problems created by the apparent existence of two orders of Reality, a physical and a psychical, which again at least seem to stand in reciprocal interaction. In the present Book we shall discuss some of the leading characteristics which everyday thought and scientific thought respectively assign to the physical order, and shall ask how these characteristics compare with those we have seen ground to ascribe to Reality, _i.e._ we shall attempt to form a theory of the place of physical existence in the whole system of Reality. In the Fourth Book we shall discuss in the same way some of the leading characteristics of the psychical order as currently conceived, and the nature of its connection with the physical order. Our treatment of these topics will necessarily be imperfect and elementary for more reasons than one: not only are the facts of which some account must be taken so numerous and complicated that they would require for their mastery something like an encyclopædic acquaintance with the whole range of the experimental sciences, physical and psychological, but their adequate interpretation, especially on the cosmological side, would demand a familiarity with the ultimate foundations of mathematical theory which is rarely possessed either by the experimentalist or by the metaphysician. The utmost we can hope to accomplish in this part of our work is to establish one or two broad results as regards general principles: any suggestions we may make as to the details of interpretation must be avowedly tentative.