Excerpt:

And there is yet another difficulty in our work from which those who cultivate physical science are happily free. They have, as the ultimate sources of their knowledge, the 'five senses' with which we are all endowed, and which are the only generally recognised inlets through which the truth of external nature can penetrate into consciousness. But we of this Society have perforce to deal with cases in which not merely the normal five or six senses, but some abnormal and half-completed sense, so to speak, comes into play; in which we have to work, not with the organisations of an ordinary and normal type, but with certain exceptional organisations who can neither explain, account for, nor control the abnormal powers they appear to possess.