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.
We have also concluded that this connecting link is likely to consist of matter in some curious condition such as to invest it with properties unlike those of ordinary matter. If on turning again to the realm of observation, we find that this deduction is substantiated in practice, we shall receive distinct confirmation of the correctness of our working hypothesis.
In the pages which follow I propose to show that there are a number of facts which strongly indicate, even if they cannot at present be held conclusively to demonstrate, the existence of some such connecting link.
I am well aware that there are numerous gaps in the body of evidence which I shall bring forward on this subject. To some of these I shall draw specific attention in the hope that by doing so I may induce some of my readers to experiment on the points in question. There is an enormous amount of research work to be done before we shall be able to have any considerable confidence in our speculations or to feel that we are working on anything like a firm foundation. Much of the evidence to which I shall refer in this chapter is in urgent need of confirmation and there is very little indeed which I should care to guarantee personally. Still the indications, slight though they are, do seem to point rather in the same direction and as my object is to stimulate investigation and, perhaps to indicate some of the lines on which it may profitably proceed rather than to lay down the law on obscure points, I have thought it worth while to deal with them fairly fully.
Historically the first relevant experiments were probably those of Reichenbach in the middle of last century. But so little was known in those days about a variety of factors which might have vitiated his results, and his work has been so strongly criticised by later authorities that I will not do more than mention him for the benefit of any reader who may have a fancy for probing into the historical origins of the subject. None the less great credit is due to Reichenbach for the thorough and painstaking character of his researches to which he brought immense industry and a truly scientific spirit which led him to fantastic and erroneous conclusions only because he had not our present knowledge to guard him from the many pitfalls which abound in these investigations.
The first phenomena to which I wish to call attention is that known as Exteriorisation of Sensibility.