§ 1. In the preceding chapter we have very briefly indicated the nature of the steps by which reflective thought comes to distinguish sharply between a physical and a psychical order of existence. The physical order, when the concept has been brought into its complete shape by the inclusion of my own body and all its parts, is thought of as a system comprising all the bodies in the universe, that is, all the existences which are of the same kind as those which I directly perceive by means of the special senses.[119] Now, with regard to the whole physical order thus conceived two things seem fairly obvious upon the least reflection, that it does not depend for its _existence_ upon the fact of my actually perceiving it, and that it does depend upon my perception for all the _qualities_ and _relations_ which I find in it. Its _that_ appears independent of the percipient, but its _what_, on the other hand, essentially dependent on and relative to the structure of the perceiving organ. As we have already seen, the familiar experience of the variations in perception which accompany differences in the permanent structure or temporary functioning of the organs of sense led, very early in the history of Philosophy, to the recognition of this relativity, so far as the so-called “secondary” qualities, _i.e._ those which can only be perceived by one special sense-organ, are concerned. We have also seen sufficiently (in Bk. II. chap. 4) that the same consideration holds equally good of those “primary” qualities which are perceptible by more senses than one, and have probably for that reason been so often supposed to be unaffected by this relativity to a perceiving organ.