What I think is clearly untenable is the view, sometimes urged by Dr. Watson, that when we are, as we think, seeing imaginary pictures with the eyes shut, we are really only using such words as would describe them. It seems to me as certain as anything can be that, when I visualise, something is happening which is connected with the sense of sight. For example, I can call up quite clear mental pictures of the house in which I lived as a child; if I am asked a question as to the furniture of any of the rooms in that house, I can answer it by first calling up an image and then looking to see what the answer is, just as I should look to see in an actual room. It is quite clear to me that the picture comes first and that words after; moreover, the words need not come at all. I cannot tell what is happening in my retina or optic nerve at these moments of visualisation, but I am quite sure that something is happening which has a connection with the sense of sight that it does not have with other senses. And I can say the same of aural and tactual images. If this belief were inconsistent with anything else that seems to me equally certain, I might be induced to abandon it. But so far as I can see, there is no such inconsistency. It will be remembered that we decided in favour of perceptions as events distinct from those which they perceive, and only causally connected with them. There is, therefore, no reason why association should not work in this region as well as in the region of muscles and glands; in other words, there is no reason to deny what used to be called “association of ideas”, in spite of the fact that bodily changes can also be associated. If a physical basis is wanted, it can be assumed to exist in the brain. The state of the brain which causes us to hear the word “Napoleon” may become associated with the state of brain which causes us to see a picture of Napoleon, and thus the word and the picture will call each other up. The association _may_ be in the sense-organs or nerves, but may equally well be in the brain. So far as I know, there is no conclusive evidence either way, nor even that the association is not purely “mental.” When we try to find a definition of the difference between a sensation and an image, it is natural to look first for intrinsic differences. But intrinsic differences between ordinary sensations and ordinary images, for example as to “liveliness”, are found to be subject to exceptions, and therefore unsuitable for purposes of definition. Thus we are brought to differences as to causes and effects.