In general, then, I should say that the phenomena of the apparent penetration of matter by matter are not established with the same degree of certainty which characterises certain other phenomena, and which we ought to demand before accepting them as scientifically proven or utilising them without reserve as a basis for the construction of theories. In the interests of the science it is in the highest degree important that experiments of this nature should be carried out under real test conditions. Should any of my readers be so fortunate as to be acquainted with any medium capable of producing these very rare phenomena with regularity, I should esteem it a great favour if they would kindly inform me. I would very much like to arrange some definite experiments to settle the matter--if possible once and for all. There is one other direction from which, in my opinion, we receive a strong hint that four-dimensional space is intimately connected with Psychic phenomena. I refer to Crawford's work on table levitation. This investigation is undoubtedly destined to take rank as a "classical" research of the first magnitude and no one who professes to take an intelligent interest in the scientific and experimental aspects of Psychic investigations can afford to be without his book.[4] In a later chapter I shall have occasion to refer to certain aspects of his results and to show how they fit in with those of other investigators working on very different lines. In the present context I propose only to call attention to the rigidity of his "cantilever," a phrase which perhaps needs some explanation. As a result of the most careful and painstaking researches extending over a period of nearly three years and performed under conditions which were singularly favourable for observation, he has been enabled to arrive at certain definite conclusions as to the mechanical causes of telekinesis in general and table levitation without contact in particular. He finds that when the table is lifted clear of the floor it is supported by a definite structure or cantilever. This structure is invisible and impalpable, or nearly so, and appears to be organised out of some form of matter actually taken from the body of the medium. Dr. Crawford has been able to work out the form and size of this structure with considerable accuracy. For the details of method and results the reader should consult his book. It is possible to pass a thin rod through this structure in any direction without causing a breakdown, and without encountering any perceptible resistance. Nevertheless the structure can resist compressional, tensional and torsional stresses of very considerable magnitude as I am able to testify from personal experience.