exactly the same way as the image in a looking-glass is related to that of which it is the reflexion; 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.
The true analysis of a sensation or idea is as follows. The element that is common to them all, and which I have called ‘consciousness,’ really is consciousness. A sensation is, in reality, a case of ‘knowing’ or ‘being aware of’ or ‘experiencing’ something. When we know that the sensation of blue exists, the fact we know is that there exists an awareness of blue. And this awareness is not merely, as we have hitherto seen it must be, itself something distinct and unique, utterly different from blue: it also has a perfectly distinct and unique relation to blue, a relation which is not that of thing or substance to content, nor of one part of content to another part of content. This relation is just that which we mean in every case by ‘knowing’. To have in your mind ‘knowledge’ of blue, is not to have in your mind a ‘thing’ or ‘image’ of which blue is the content. To be aware of the sensation of blue is not to be aware of a mental image—of a ‘thing,’ of which ‘blue’ and some other element are constituent parts in the same sense in which blue and glass are constituents of a blue bead. It is to be aware of an awareness of blue; awareness being used, in both cases, in exactly the same sense. This element, we have seen, is certainly neglected by the ‘content’ theory: that theory
