The behaviourist does not admit images, but he equally does not admit sensations and perceptions. Although he does not say so quite definitely, he may be taken to maintain that there is nothing but matter in motion. We cannot, therefore, tackle the question of images by contrasting them with sensations or perceptions, unless we have first clearly proved the existence of these latter and defined their characteristics. Now it will be remembered that in Chapter V we attempted a behaviourist definition of perception, and decided that its most essential feature was “sensitivity”. That is to say, if a person always has a reaction of a certain kind B when he has a certain spatial relation to an object of a certain kind A, but not otherwise, then we say that the person is “sensitive” to A. In order to obtain from this a definition of “perception”, it is necessary to take account of the law of association; but for the moment we will ignore this complication, and say that a person “perceives” any feature of his environment, or of his own body, to which he is sensitive. Now, however, as a result of the discussion in Chapter XVI, we can include in his reaction, not only what others can observe, but also what he alone can observe. This enlarges the known sphere of perception, practically if not theoretically. But it leaves unchanged the fact that the essence of perception is a causal relation to a feature of the environment which, except in astronomy, is approximately contemporaneous with the perception, though always at least slightly earlier, owing to the time taken by light and sound to travel and the interval occupied in transmitting a current along the nerves.