Without wasting the reader’s time by unnecessary repetition of our former reasoning, it may be worth while to point out here how this thorough-going relativity of the qualities of the physical order to a percipient organ leads directly to the indefinite regress, the apparently invariable consequence of all contradictions in Metaphysics, when we try to take those qualities as independently real. I perceive the properties of physical existence by special sense-organs, and the properties as perceived are conditioned by the structure of those organs. But each sense-organ is itself a member of the physical order, and as such is perceived by and dependent for its perceived qualities upon another organ. This second sense-organ in its turn is also a member of the physical order, and is perceived by a third, or by the first organ again. And there is no end to this mutual dependence. The physical order, as a whole, must be a “state” of my nervous system, which is itself a part of that order. We shall see more fully in our final Book, when we come to discuss the problem of Mind and Body, that this contradiction is an inevitable result of the inconsistency involved in the inclusion of my own body in the physical order, an inconsistency which is, in its turn, a necessary consequence of the hard-and-fast separation of the two orders of existence.[120] Considerations of this kind have led to the general recognition that the physical order must be regarded as phenomenal, as the manifestation to sense-perception of a reality which is in its own nature inaccessible to sense-perception, and therefore, in the strictest sense of the words, not physical. When we ask, however, how this non-physical reality of which the physical order is the phenomenal manifestation to our senses, is to be thought of, we find ourselves at once plunged into the same difficulties which we have already met, in a more general form, in discussing the concept of Substance. Popular thought, and science so far as it is content to accept the notions of popular thought without criticism, have commonly fallen back on the idea of the non-phenomenal ground of the physical order as an unperceived “substratum.” To this substratum it has given the name of _matter_, and has thus interpreted the physical order as the effect produced by the causal action of an unperceived matter upon our sense-organs, or rather, to speak with more precision, upon _their_ unknown material substratum. Frequently, as might have been expected, the attempt has been made to identify this substratum with those of the known qualities of the physical order which appear least liable to modification with the varying states of the percipient organs, and lend themselves most readily to measurement and calculation, the so-called “primary” qualities of mechanical science. This is the standpoint adopted by Newton and, in the main, by Locke, and largely through the influence of their work still remains the most