"On the first exercise of philosophic thought, however, this conception is blurred. An immense quantity of what we all instinctively think of as really existing out of ourselves turns out, on investigation, not to exist at all as we fancy it existing, but to consist only of affections of the perceiving mind. The redness of the rose is not a real external thing, immutably the same in itself; it is only a certain peculiar action on my physiology which the presence of an external cause or object seems to determine. Were my physiology different, the action would be different, though the cause or object remained the same. Indeed, there _are_ persons in whom the presence of a rose occasions no sensation of redness such as is known to me, but a much vaguer sensation, not distinguishable from what I should at once distinguish as greenness. And, as colour is thus at once detected as no external independently-existing reality, but only a recurring physiological affection of myself and other sentient beings like myself, so with a thousand other things which, by habit or instinct, I suppose as externally and independently existing. When I imagine the depopulated Earth still wheeling its inanimate rotundity through the daily sunshine and the nocturnal shadow, or one of its bays still resonant in moonlit evenings with the roar of the breaking waves, it is because, in spite of myself, I intrude into the fancy the supposition of a listening ear, and a beholding eye analogous to my own. It is only by a strong effort that I can realize that a great deal at least of what I thus think of as the goings-on of things by themselves is not and cannot be their goings-on by themselves, but consists at the utmost of effects interbred between them and a particular sentiency in the midst of them. But the effort may be made; and, when it is made repeatedly, in a great many directions, and with reference to a great many of the so-called properties of matter, the inevitable result for the philosophic mind is that the popularly-imagined substance of a real external world finds itself eaten away or corroded, at least to a certain depth. So far philosophers are agreed. It is when they proceed to consider to _what_ depth the popularly-imagined substance of the real external world is thus eaten away, or accounted for, that they begin to differ.