We have then in every sensation two distinct elements, one which I call consciousness, and another which I call the object of consciousness. This must be so if the sensation of blue and the sensation of green, though different in one respect, are alike in another: blue is one object of sensation and green is another, and consciousness, which both sensations have in common, is different from either. But, further, sometimes the sensation of blue exists in my mind and sometimes it does not; and knowing, as we now do, that the sensation of blue includes two different elements, namely consciousness and blue, the question arises whether, when the sensation of blue exists, it is the consciousness which exists, or the blue which exists, or both. And one point at least is plain: namely that these three alternatives are all different from one another. So that, if any one tells us that to say 'Blue exists' is the _same_ thing as to say that 'Both blue and consciousness exist,' he makes a mistake and a self-contradictory mistake. But another point is also plain, namely, that when the sensation exists, the consciousness, at least, certainly does exist; for when I say that the sensations of blue and of green both exist, I certainly mean that what is common to both and in virtue of which both are called sensations, exists in each case. The only alternative left, then, is that _either_ both exist or the consciousness exists alone. If, therefore, any one tells us that the existence of blue is the same thing as the existence of the sensation of blue he makes a mistake and a self-contradictory mistake, for he asserts _either_ that blue is the same thing as blue together with consciousness, _or_ that it is the same thing as consciousness alone. Accordingly to identify either "blue" or any other of what I have called "_objects_" of sensation, with the corresponding sensation is in every case, a self-contradictory error. It is to identify a part either with the whole of which it is a part or else with the other part of the same whole. If we are told that the assertion "Blue exists" is _meaningless_ unless we mean by it that "The sensation of blue exists," we are told what is certainly false and self-contradictory. If we are told that the existence of blue is inconceivable apart from the existence of the sensation, the speaker _probably_ means to convey to us, by this ambiguous expression, what is a self-contradictory error. For we can and must conceive the existence of blue as something quite distinct from the existence of the sensation. We can and must conceive that blue might exist and yet the sensation of blue not exist. For my own part I not only conceive this, but conceive it to be true. Either therefore this terrific assertion of inconceivability means what is false and self-contradictory or else it means only that _as a matter of fact_ blue never can exist unless the sensation of it exists also.