The reliability of witnesses is a crucial question in the study of psychical phenomena and has for long been a bone of contention between spiritualists and their critics. If honesty, care, and intelligence alone sufficed to make a man’s testimony reliable the whole range of spiritualistic phenomena, including spirit photography, might long ago have been taken as proved beyond all possibility of doubt. But this is very far from being the case, and although it is never pleasant to express flat disbelief of the accuracy of people’s statements, the Psalmist’s dictum that “all men are liars” should be graven on the heart of every psychical researcher, especially in the case of those who attempt to investigate “physical” phenomena.[11] I do not propose to repeat the obvious platitudes about the ease with which conjurers can deceive their audiences, but I should like to emphasise the fact that such differences as exist between the circumstances in which conjurers and mediums work are uniformly in favour of the latter as regards the minor manipulations necessary for the production of photographic phenomena. (One is not, of course, concerned with elaborate “stage effects,” but rather with small matters like the substitution of one plate for another or the distraction of the sitter’s attention while the required extra is impressed upon the plate.) The conjurer’s audience _knows_ that it is a trick; the medium’s does not. Even the most hardened sceptic will probably have a lingering doubt in his mind as to whether there may not possibly be “something in it” after all. This is all to the medium’s advantage, and it must be remembered that not only does he work for much of his time under lighting conditions which are peculiarly favourable to fraudulent manipulation, but also that the great majority of his sitters start with a considerable prepossession to the effect that they are encountering something inexplicable. But these observations must, I suppose, have occurred to all who have considered such matters at all impartially, and however relevant they may be they will never by themselves prevail against what we call “the evidence of our senses.” No amount of general considerations of this kind will deter the credulous from accepting the _prima facie_ indications of a “successful” _séance_. The only hope of preserving the public from the depredations of these swindlers is to show that the “evidence of the senses” is not worth twopence unless backed by special knowledge of the relevant technique.