_The 'backs' of Things._ We talk of the back of a thing, but nobody has ever seen a back. Things have no backs in the popular sense of the word. When we turn round a back to perceive it, it is then a front. Everything is a flat upright surface, and its appearance of solidity can be imitated on a surface known to be flat, and with nearly the same illusive completeness as in the original object. In turning things round we merely change the surface; we are exercising our power to alter primary consciousness. When two persons perceive the 'same' object from contrary directions, the sameness means that the two objects proceed from the same cause, or can be reduced to the same general idea. But the objects are numerically distinct. By a similar turn of speech we say that A and _a_ are the same _letter_, but they are evidently distinct and dissimilar objects. If we hold a thing before a mirror and see what is termed its back, we produce a new object resembling the first in some respects but without its resistance. Resistance is a negative term signifying the limit of our power to alter primary experience. Where our power ceases resistance is said to begin, and we meet with resistance when we apply a power inadequate to the desired effect. Dr. Johnson's solitary experiment in idealistic philosophy has been often related. He struck a post, and because it did not disappear he thought he had disproved Berkeley's statement that material objects exist only in the perceiving mind. The experiment merely showed that all means are not adequate to change all primary experience. Had he shut his eyes, or turned a corner, or occupied his attention with other matters, the post would have vanished. He chose improper means and therefore met with 'resistance.' No idealist believes we can change our primary experience by any capricious and frivolous means[15]. _Geographical Concretion._ The knowledge of large geographical areas is an artificial construction without objective reality.