Yet unfamiliar causes must be introduced occasionally into systematised knowledge, unless our scrutiny of the Universe is already exhaustive. Unpalatable facts can be ruled out from attention, but they cannot without investigation be denied. Strange facts do really happen, even though unprovided for in our sciences. Amid their orthodox relations, they may be regarded as a nuisance. The feeling they cause is as if capricious or mischievous live things had been allowed to intrude into the determinate apparatus of a physical laboratory, thereby introducing hopeless complexity and appearing superficially to interfere with established laws. To avoid such alien incursion a laboratory can be locked, but the Universe can not. And if ever, under any circumstances, we actually do encounter the interaction of intelligences other than that of living men, we shall sooner or later become aware of the fact, and shall ultimately have to admit it into a more comprehensive scheme of existence. Early attempts, like those of the present, must be unsatisfactory and crude; especially as the evidence is of a kind to which scientific men for the most part are unaccustomed; so no wonder they are resentful. Still the evidence is there, and I for one cannot ignore it. Members of the Society for Psychical Research are aware that the evidence already published--the carefully edited and sifted evidence published by their own organisation--occupies some forty volumes of _Journal_ and _Proceedings_; and some of them know that a great deal more evidence exists than has been published, and that some of the best evidence is not likely to be published,--not yet at any rate. It stands to reason that, at the present stage, the best evidence must often be of a very private and family character. Many, however, are the persons who are acquainted with facts in their own experience which appeal to them more strongly than anything that has ever been published. No records can surpass first-hand direct experience in cogency. Nevertheless we are also aware, or ought to be, that no one crucial episode can ever be brought forward as deciding such a matter. That is not the way in which things of importance are proven. Evidence is cumulative, it is on the strength of a mass of experience that an induction is ultimately made, and a conclusion provisionally arrived at; though sometimes it happens that a single exceptionally strong instance, or series of instances, may clinch it for some individual.