From these experimental cases we may pass to the consideration of spontaneous hallucinations, and amongst them to that class with which we are more directly concerned, the occasional hallucinations of sane and healthy persons. Owing, amongst other causes, to their comparative infrequency, and to the difficulty of obtaining accurate contemporary records (since their occurrence cannot, as in the hallucinations of disease, be foreseen), phenomena of this class have hitherto attracted little attention amongst psychologists.[98] Mr. Edmund Gurney, however, in 1884 and onwards conducted an inquiry, by means of a printed schedule of questions, amongst a circle of some 6000 persons; and during the last four years, at the request of the Congress of Experimental Psychology which met at Paris in 1889, Professor Henry Sidgwick, with the aid of a Committee of members of the S.P.R., has carried on a similar investigation on a larger scale. 17,000 adult persons, for the most part resident in the United Kingdom, have been questioned as to their experience of sensory hallucinations.[99] In the result it appeared that 1684 out of 17,000, or 9.9 per cent.--to wit, 655 out of 8372 men, and 1029 out of 8628 women--had experienced a sensory hallucination at some time in their lives. In about one-third of the cases the percipient had more than one experience of the kind. The phenomenon, therefore, though not so common as dreaming, is less rare than is generally supposed, seeing that about one in every ten educated persons has such an experience in the course of his life. The inquiries of the Committee have revealed no general cause for the greater number of these isolated hallucinations. In a small proportion of the cases there was a slight degree of ill-health, and in a rather greater number there was a certain amount of anxiety or other emotional excitement, to which the hallucinatory experience might with some plausibility be attributed.[100] But in the great majority of the cases there was no obvious antecedent to be discovered either in the condition of the percipient or in the surrounding circumstances, and we are led to the conclusion that an isolated hallucination of this kind is as little incompatible with ordinary health as a blush or a hiccough. At the same time we are entitled to infer, from the relatively large proportion of cases occurring when the percipient is in bed, or alone, that quiescence and freedom from external stimuli are favourable conditions for the genesis of hallucinations.[101] They may, in short, be regarded as unusually vivid dreams, and have for the most part just so much interest and significance. The nature and variety of these casual hallucinations may be gathered from the table on the following page.