We know that there must be some form of connection because impressions which are received by the three-dimensional sense organs are transmitted to the conscious Ego, which is, _ex hypothesi_, embodied in the four-dimensional vehicle. Furthermore it is clear that the connection can be interrupted with comparative ease, since in sleep, anæsthesia, and analogous conditions, the conscious Ego does not receive these impressions although the sense organs may still be subject to stimuli to a greater or less degree. We are not, of course, able to draw detailed conclusions as to the precise nature of this connection by the exercise of pure deductive reason. But I think that my readers will agree with me that the first and most obvious place to look for it will be in the realm of the nervous system. Further we may safely say that, assuming the hypothesis we are considering to be correct, the sense impression must, at some stage in its transmission, be deflected, so to speak, out of three space into four space. In order for this to happen it is necessary that some part of the transmitting mechanism should be capable of producing this deflection and it is reasonable to suppose that a substance or mechanism specially differentiated for the purpose of deflecting impressions in this manner out of three space into four space, will be distinguished by an abnormal four-dimensional complexity as compared with ordinary matter, which, as we have already seen, probably possesses a very slight four-dimensional extension. As a result of this abnormal four-dimensional complexity it is to be anticipated that the part of the transmitting mechanism concerned will possess characteristics sufficient to differentiate it from ordinary matter. I submit, then, that we may reasonably deduce that if the four-dimensional hypothesis which I have outlined be correct, there should exist, either as an integral part of the nervous system or in close association with it, some constituent or substance which, in spite of having many of the properties of ordinary matter, will also possess characteristics peculiar to itself--as, for instance, susceptibility to four-dimensional forces imperceptible to us. At this point I would recall to the reader's attention the remarks which I made in Chapter II regarding the processes of scientific thought and the sequence of operations whereby we attain to exact knowledge. So far we have considered a number of observed facts and framed a working hypothesis which, I believe, explains some, and is not contradicted by any, of them. In the immediately preceding paragraphs we have, by deductive reasoning, concluded that if this hypothesis be correct then something else must follow. There must, in fact, be some sort of connecting link whereby sense impressions are deflected out of three space into four space and are thus enabled to get through to the consciousness.