J. DELBCEUF, LE SOMMEIL ET LES REVES. 117

between the percept and the image, we could never have learnt to distinguish the two under ordinary conditions, and so he has to fall back on the crude distinction drawn by Hume and others, viz., the superior vivacity of the percept. Assuming this to be an all-present and sufficient mark of the percept, he follows M. Taine, to whom however he does not refer, in regarding the illu- sion of the dream as due to the suppression of the more vivid mental states excited by external objects. We believe in the reality of our dream-images, not because they differ in absolute degree of vivacity from ordinary images, but because, owing to the exclusion of external impressions,, they have gained enormously in relative force. I am not quite sure that I fully understand Prof. Delbceuf here. He can hardly mean, I fancy, that in the state of sleep images do not persist and master the attention with a force incomparably greater than that of waking images, even when, as in shutting the eyes in a quiet room, the effect of exter- nal impressions is very greatly reduced. The vividness and dis- tinctness of detail with which one is often able to recall a dream immediately after waking, and when the fresh impression of the external world is particularly powerful, points, I think, unmis- takably to the absolute vivacity of the dream-image. To say that the image can only attain to this degree of vivacity on the condition that external impressions are withdrawn is one thing ; to say that it has only gained in relative vivacity is another. Prof. Delboeuf, in discussing the criterion of true perception, appears to make far too little of the coherent testimony of the different senses. Also, he writes hastily when he says that he only knows of one sense that is capable of correcting the others, viz,, touch for it is a familiar fact that we rid ourselves of the momentary illusion due to a subjective skin- sensation by a glance of the eye. No doubt, as he says, the most important criterion is the consensus between the impressions of the individual and the testimony of others ; but even this, as he virtually admits, is not uniformly conclusive, for, given a multitude of men sub- jected to the same disturbing conditions of panic, a common illu- sion becomes not only possible but probable. The result of this inquiry into the grounds of certitude is that there is no absolute criterion of truth. At the same time we are able to reach a reasonable degree of certainty, which speculative doubt, essentially insincere, is wholly unable to disturb, and of which indeed this so-called doubt is a sufficient distinctive sign. After dealing with the logical side of the dream, Prof. Delboeuf discusses its psychological origin, and more particularly its rela- tion to memory. He here sets out with a full account of a curi- ous dream of his own in which, among other products of past experience, habits of life, &c., there occurred a botanical name which upon waking appeared to be quite unfamiliar to him. It was many years after that the puzzle was explained by his find- ing the word in a herbarium that some friends of his had brought