it, the coincidence would deserve far less consideration. And yet the cerebral mechanism involved in both processes is no doubt very similar. A dream is a hallucination in sleep, and a hallucination is only a waking dream; though it is probable that the waking impression, seeing that it can contend on equal terms with the impressions derived from external objects, is more vivid than the common run of dreams. But the evidence of dream coincidence is defective, primarily, from the frequency of dreams; it is only a small proportion of educated persons, at any rate, who ever experience a hallucination, but everybody dreams occasionally, and some persons dream every night. Clearly there must be here a wide scope for coincidence. Secondly, whilst dream impressions are probably less vivid at the time, they are certainly more elusive in the memory. There is a serious risk, therefore, that after the event is known detailed correspondences may be read back into the indistinct picture preserved in the memory; or that a dream which at the time made but a slight impression may be charged retrospectively with emotional significance. Finally, as the dream does not enter into any organic series of impressions, and has no landmarks of its own, either in space or time, it becomes after the lapse of a few days, or even hours, a matter of difficulty to determine its date. Against the last two sources of error it is indeed possible to guard. Under ordinary circumstances no dream should be regarded as having evidential value which has not been either recorded in writing or mentioned to some other person before the coincidence is known. Mention of the dream immediately after the receipt of the news, even with persons of proved accuracy, can by no means be regarded as equivalent to mention of it beforehand. For it is possible, as already pointed out (p. 155), that some alleged coincident and prophetic dreams may be due to hallucination of memory, or still more probably to the embellishment and amplification of vague pre-existent memories.