These difficulties are much reduced, though not entirely removed, if we suppose that the agent is embodied, not in the four-dimensional vehicle, but in what, for lack of a better word, is called the "Etheric Double." This appears to be of a semi-material nature and is discussed at length in the chapter dealing with "The Connecting Link." But this supposition would involve special difficulties of its own. There is reason to suppose that the "Etheric Double," if it exists at all, is incapable of moving far from the physical body during life and it does not appear well adapted for use as a vehicle after death. But on this point I shall have more to say later. Generally speaking, it seems probable that no one of these explanations will be found to cover all the cases in question. But each is likely to prove applicable to some of them, although much careful investigation and analysis will be necessary before we can hope to be able to allot each case to its true cause with any degree of assurance. None the less I am convinced that the hypothesis of a four-dimensional vehicle, detachable on occasion from the physical body, puts us, at least, on the right track. I will now turn to the consideration of a series of phenomena which, from the point of view of the higher space hypothesis, are of far greater interest and significance than any we have yet considered. I refer to the phenomena of "apport" and of "apparent penetration of matter by matter." If we have a closed room, of which all the windows, doors, and other apertures have been carefully shut and sealed, it is clearly impossible to introduce any solid object into that room, by normal means, without breaking the seals and opening one of the apertures. The same would apply to a closed, locked and sealed box. But the literature of Psychical research abounds with instances where objects are alleged to have been introduced into such closed and sealed rooms and boxes--or removed from them, which comes to the same thing--_without_ breaking the seals. This is the phenomenon of "apport" properly so called and it forms a special case of the more general class of "apparent penetration of matter by matter." Other cases of the latter are the tying of knots in an endless cord of such a nature that they can only be untied by breaking the cord or separating its previously sealed ends; or the passing, on to the wrist or ankle of some person or other, of a ring so small that it could not possibly be pushed on over the hand or foot. A very good test would be the interlinking of two rings turned from different sorts of wood--as was attempted without success in the Slade-Zöllner investigation; or the passing of a piece of weldless drawn steel tube on to the middle portion of an ordinary wooden dumb-bell. With regard to these phenomena I propose, first, to show in what their very great importance lies and then to discuss the nature of the evidence we have for their actual occurrence.