Visual perception supplies numberless instances in which the same sensation of vision is perceived as one object or another according to the interpretation of the mind. Many of these instances will come before us in the course of the next two chapters; and in Chapter XIX similar illusions will be described in the other senses. Taken together, all these facts would force us to admit that _the subjective difference between imagined and felt objects is less absolute than has been claimed, and that the cortical processes which underlie imagination and sensation are not quite as discrete as one at first is tempted to suppose. That peripheral sensory processes are ordinarily involved in imagination seems improbable; that they may sometimes be aroused from the cortex downwards cannot, however, be dogmatically denied._ * * * * * _The imagination-process_ CAN _then pass over into the sensation-process._ In other words, genuine sensations _can_ be centrally originated. When we come to study hallucinations in the chapter on Outer Perception, we shall see that this is by no means a thing of rare occurrence. At present, however, we must admit that _normally the two processes do_ NOT _pass over into each other_; and we must inquire why. One of two things must be the reason. Either 1. Sensation-processes occupy a different _locality_ from imagination-processes; or 2. Occupying the same locality, they have an _intensity_ which under normal circumstances currents from other cortical regions are incapable of arousing, and to produce which currents from the periphery are required.