We all know that the sensation of blue differs from that of green. But it is plain that if both are sensations they also have some point in common. What is it that they have in common? And how is this common element related to the points in which they differ?
I will call the common element 'consciousness' without yet attempting to say what the thing I so call is. We have then in every sensation two distinct terms, (1) 'consciousness,' in respect of which all sensations are alike; and (2) something else, in respect of which one sensation differs from another. It will be convenient if I may be allowed to call this second term the 'object' of a sensation: this also without yet attempting to say what I mean by the word.
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.
And at this point I need not conceal my opinion that no philosopher has ever yet succeeded in avoiding this self-contradictory error: that the most striking results both of Idealism and of Agnosticism are only obtained by identifying blue with the sensation of blue: that esse is held to be percipi, solely because what is experienced is held to be identical with the experience of it. That Berkeley and Mill committed this error will, perhaps, be granted: that modern Idealists make it will, I hope, appear more probable later. But that my opinion is plausible, I will now offer two pieces of evidence. The first is that language offers us no means of referring to such objects as "blue" and "green" and "sweet," except by calling them sensations: it is an obvious violation of language to call them "things" or "objects" or "terms." And similarly we have no natural means of referring to such objects as "causality" or "likeness" or "identity," except by calling them "ideas" or "notions" or "conceptions." But it is hardly likely that if philosophers had clearly distinguished in the past between a sensation or idea and what I have called its object, there should have been no separate name for the latter. They have always used the same name for these two different "things" (if I may call them so): and hence there is some probability that they have supposed these "things" not to be two and different, but one and the same. And, secondly, there is a very good reason why they should have supposed so, in the fact that when we refer to introspection and try to discover what the sensation of blue is, it is very easy to suppose that we have before us only a single term. The term "blue" is easy enough to distinguish, but the other element which I have called "consciousness"—that which sensation of blue has in common with sensation of green—is extremely difficult to fix. That many people fail to distinguish it at all is sufficiently shown by the fact that there are materialists. And, in general, that which makes the sensation of blue a mental fact seems to escape us: it seems, if I may use a metaphor, to be transparent—we look through it and see nothing but the blue; we may be convinced that there is something but what it is no philosopher, I think, has yet clearly recognised.
But this was a digression. The point I had established so far was that in every sensation or idea we must distinguish two elements, (1) the "object," or that in which one differs from another; and (2) "consciousness," or that which all have in common—that which makes them sensations or mental facts. This being so, it followed that when a sensation or idea exists, we have to choose between the alternatives that either object alone, or consciousness alone, or both, exist; and I showed that of these alternatives one, namely that the object only exists, is excluded by the fact that what we mean to assert is certainly the existence of a mental fact. There remains the question: Do both exist? Or does the consciousness alone? And to this question one answer has hitherto been given universally: That both exist.
This answer follows from the analysis hitherto accepted of the relation of what I have called "object" to "consciousness" in any sensation or idea. It is held that what I call the object is merely the "content" of a sensation or idea. It is held that in each case we can distinguish two elements and two only, (1) the fact that there is feeling or experience, and (2) what is felt or experienced; the sensation or idea, it is said, forms a whole, in which we must distinguish two "inseparable aspects," "content" and "existence." I shall try to show that this analysis is false; and for that purpose I must ask what may seem an extraordinary question: namely what is meant by saying that one thing is "content" of another? It is not usual to ask this question; the term is used as if everybody must understand it. But since I am going to maintain that "blue" is not the content of the sensation of blue, and what is more important, that, even if it were this analysis would leave out the most important element in the sensation of blue, it is necessary that I should try to explain precisely what it is that I shall deny.
What then is meant by saying that one thing is the "content" of another? First of all I wish to point out that "blue" is rightly and properly said to be part of the content of a blue flower. If, therefore, we also assert that it is part of the content of the sensation of blue, we assert that it has to the other parts (if any) of this whole the same relation which it has to the other parts of a blue flower—and we assert only this: we cannot mean to assert that it has to the sensation of blue any relation which it does not have to the blue flower. And we have seen that the sensation of blue contains at least one other element beside blue—namely, what I call "consciousness," which makes it a sensation. So far then as we assert that blue is the content of the sensation, we assert that it has to this "consciousness" the same relation which it has to the other parts of a blue flower: we do assert this, and we assert no more than this. Into the question what exactly the relation is between blue and a blue flower in virtue of which we call the former part of its "content" I do not propose to enter. It is sufficient for my purpose to point out that it is the general relation most commonly meant when we talk of a thing and its qualities; and that this relation is such that to say the thing exists