A great many theories have been put forward by psychologists and others to account for this paramnesic phenomenon. The most ancient explanation, long anterior to the beginnings of scientific psychology, was the theory that the occurrence which, as it now happens, strikes us as so overwhelmingly familiar had actually occurred to us in a previous existence long ages before; thus Pythagoras, according to the ancient story, when he visited the temple of Juno at Argos recognised the shield he had worn ages before when he was Euphorbus and fought with Menelaus in the Trojan war. A much more recent theory runs to the opposite extreme and claims that all or nearly all these cases of recognition indicate a real but confused reminiscence of past events in our present life, dim recollections which the subject is unable definitely to locate. This is the explanation largely relied on by Ribot, Jessen, Sander, Sully, Burnham, and many others. It was perhaps largely due to ignorance of the phenomenon; Ribot, when he wrote his book on the diseases of memory, considered that only three or four cases had been recorded, for an abnormal phenomenon always seems rare until it is recognised and definitely searched for. Undoubtedly, this theory will explain a considerable proportion of cases, but not really typical cases in which the subject has an overwhelming conviction that even the minute details of the present experience have been experienced before. We may read a new poem with a vague sense of familiarity, but such an experience never puts on a really paramnesic character, for we quickly realise that it is explainable by the fact that the writer of the poem has fallen under the influence of some greater master. The only experience I can personally speak of as at all approaching true paramnesia occurred on visiting the ruins of Pevensey Castle many years ago. On going up the slope towards the ivy-covered ruins, bathed in bright sunlight, I experienced a strange and abiding sense of familiarity with the scene. Three theories might account for this experience (for I refrain from including the Pythagorean theory that I experienced a reminiscence of the experience of a possible ancestor coming from across the Thames to the assistance of Harold against William the Conqueror at this spot): (1) that it was a case of true paramnesia; (2) that I had been taken to the spot as a child; (3) that the view was included among a series of coloured stereoscopic pictures with which I was familiar as a child, and which certainly contained similar scenes. I incline to this last explanation. Here, as elsewhere, there are no keys which will unlock all doors.