At first there was no means of determining directly whether this was so or not. But later the researches of Foucault made it possible to settle the point by direct measurement. When the velocity of light in air and water respectively was measured directly by Foucault's method it was found that the velocity in water was _less_ than that in air. The Corpuscular theory was therefore untenable. It is only by this process of forming, testing and, if necessary, rejecting hypotheses that we gradually attain to exact knowledge. As Prof. Richet says: "La science n'a jamais été qu'une serie d'erreurs, approximations constamment evoluant constamment boulversé, et cela d'autant plus vite qu'elle était plus avancée." (Annales des sciences psychiques, 1905, p. 15.) From this brief resumé of the steps involved in scientific progress it is clear that the formation of a working hypothesis, by inductive reasoning from the observed facts, is a normal, necessary, and invariable step in the progress of any science whatsoever. For this reason I do not think it likely that Psychical research will attain any widespread recognition as a science until it is in possession of a valid working hypothesis capable of explaining at least the more important of the observed facts. I believe that the higher space hypothesis fulfills this condition and if so it is clearly worth while adopting, purely provisionally and tentatively of course, by those who concern themselves with the subject. I have said that I think that the conception of higher space has a bearing on the relations between Religious and Scientific thought. I shall reserve for a later chapter the treatment of the question from the purely religious stand-point, and shall only examine here the reasons which seem to me to have led so many sincere and able scientific men to a position at variance with the religious and spiritual point of view. This is, of course, closely bound up with the whole topic of the various attempts which have been made to satisfy the perennial demand for light on the mysteries of life and death and on the spiritual and non-material aspects of the universe. It is out of the question for me to attempt to classify here the countless religions, sects, and philosophies which have arisen from time to time. But they do seem to fall into three main groups and although it is impossible to label these in any really satisfactory manner I think one may say that the Materialistic Scientists are the representatives of one school, the Orthodox Theologians of another, and the Occultists of a third. By the Materialistic Scientists I mean those who see in matter or ether the ultimate and only permanent reality and who attempt to explain every experienced phenomenon in terms of matter and ether and of these only.