The fact that we cannot perceive four space, or picture its nature to ourselves, is no proof that it is non-existent. I suggest as a working hypothesis that four space is a reality and that Man possesses at least one other vehicle of Consciousness--a four-dimensional one--besides his physical body. In this vehicle he is embodied after discarding the physical vehicle at death and also during temporary absences from the body during life. This hypothesis is likely to prove of importance in two respects. First, it provides Psychic Research with a working hypothesis which may be essential to its development as a science. Secondly the adoption of some such hypothesis should go far to remove the principle cause of recent cleavage between Religious and Scientific thought. The hypothesis is capable of throwing light on a number of "Psychic" phenomena which are otherwise very obscure. It affords us a means of conceiving a mode of existence which is real and yet imperceptible to our senses, thus surmounting one of the chief difficulties in the way of conceiving of post-mortem existence. In the realm of Clairvoyance it enables us to form some idea of the nature of the faculty of internal vision. With regard to Clairvoyance in space, it also helps us to some slight extent, although this phenomenon presents special difficulties of its own. Other varieties of "out of the body" experiences are much elucidated by its aid. The phenomena with which it is most closely connected, however, are those known under the general title of "apparent penetration of matter by matter." To these it affords by far the simplest and probably the only explanation and, if they are regarded as irrefutably established, it will be difficult to avoid the conclusion that four space is a reality. The _locus classicus_ of such phenomena is the Slade-Zöllner investigation, but this is worthless as evidence. The literature of the subject abounds with records of similar occurrences. The hypothesis also seems to offer a possible means of explaining the paradoxical rigidity of the impalpable structure discovered by Crawford. The hypothesis may also have a certain significance, even in the realm of pure Philosophy. It enables us to conceive of the simultaneous existence of a series of three space simultaneities and, consequently, is of interest in the consideration of Time and of the possibility of Prevision. It also works in well with a certain view of the nature of Vitality.