We have it, then, as a universally received opinion that blue is related to the sensation or idea of blue, as its _content_, and that this view, if it is to be true, must mean that blue is part of _what_ is said to exist when we say that the sensation exists. To say that the sensation exists is to say both that blue exists and that "consciousness," whether we call it the substance of which blue is _the_ content or call it another part of the content, exists too. Any sensation or idea is a "_thing,_" and what I have called its object is the quality of this thing. Such a "thing" is what we think of when we think of a _mental image._ A mental image is conceived as if it were related to that of which it is the image (if there be any such thing) in exactly the same way as the image in a looking-glass is related to that of which it is the reflection; in both cases there is identity of content, and the image in the looking-glass differs from that in the mind solely in respect of the fact that in the one case the other constituent of the image is "glass" and in the other case it is consciousness. If the image is of blue, it is not conceived that this "content" has any relation to the consciousness but what it has to the glass: it Is conceived _merely_ to be its _content._ And owing to the fact that sensations and ideas are all considered to be _wholes_ of this description--things in the mind--the question: What do we know? is considered to be identical with the question: What reason have we for supposing that there are things outside the mind _corresponding_ to these that are inside it?
What I wish to point out is (1) that we have no reason for supposing that there are such things as mental images at all--for supposing that blue _is_ part of the content of the sensation of blue, and (2) that even if there are mental images, no mental image and no sensation or idea is _merely_ a thing of this kind: that 'blue,' even if it is part of the content of the image or sensation or idea of blue, is always _also_ related to it in quite another way, and that this other relation, omitted in the traditional analysis, is the _only_ one which makes the sensation of blue a mental fact at all.