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.