Similar fusions occur in the various senses, and they become very complicated owing to the fact that sensation-elements of several senses are joined together at the same time. The disappearance of the components into one resulting product brings it about that we cannot directly perceive the separate elements that make up this product by means of direct sensation, as is in part possible in the case of clang-fusions. We are forced to make use of an indirect method. We proceed from the principle, that each sensation, a change in which is of essential influence on the resulting idea, belongs to the components of this idea. A pronounced case of this kind is seen very clearly in spatial ideas of the senses of touch and sight. If any part of the skin is touched with a little rod, we can, as is well known, with a fair degree of certainty apprehend the place touched, without looking at it. Now in the pathological cases of partial paralysis, it is shown that there are two kinds of sensations that are of essential influence on this localisation. Firstly, it is considerably disturbed by a partial suspension of the outer cutaneous sensitivity. In this case the patient often localises the impression on a place far removed from the place touched. Secondly, complete or partial paralysis of the muscles in the region of the place touched, e.g. the muscles of the arm and hand in the case of a touch sensation on the hand, causes just as much confusion in localisation. In this case as well the patient may localise the impression on an absolutely wrong part of the body. Therefore we must presuppose that neither cutaneous nor muscle sensations alone are the original cause of the idea of the place touched, but that both together by fusion give rise to this idea. After this has once happened, the quality of the touch sensation which is peculiar to each part of the skin and which varies with the place of the impression, can in itself bring about a localisation. That in general both components, i.e. cutaneous and movement sensations, must fuse together in order to produce an idea of a certain place or locality, is clearly shown in blind people, and especially in those born blind. In their case the sense of sight, which determines the whole perception of space for those who can see, is wanting, and we observe in them a continuous and very lively co-operation of cutaneous sensations and movements of touch.