§ 6. Hallucinations, Illusions, and Dreams. — Two conditions are necessary to constitute an hallucination. On the one hand, a presentation must exist, having some or all of the distinctive characteristics of actual senseperception. On the other hand, the object as it appears to be perceived must not be actually present to the senses. The specific nature of the object presented must be constituted by subjective conditions, not by the present operation of an external stimulus. Some sort of stimulation may be present and nearly always is present; but it does not determine the nature of the object presented; it only serves to give the experience a sensational character.

In illusion an object is actually present to the senses, and produces to some extent sensations such as it would normally produce: but these sensations are wronglyinterpreted. A presentation may be partly an illusion, and partly an hallucination. Thus we may see a man, when what is actually present is a suit of clothes. The special nature of the sensations experienced may be due partly to the suit of clothes, and partly to subjective conditions; so far as the sensations which arise in a normal manner from the external stimulus are wrongly interpreted, there is illusion; so far as other sensations due to subjective conditions enter into experience, there is hallucination. It may happen in such a case that no other sensations are present except those which the suit of clothes would normally produce: and that the error lies wholly in a wrong interpretation. When this is so, the illusion is a pure illusion without any element of hallucination.

It is not necessary that all the characteristics of perceptual experience should be present in hallucinations. Dreams partake of the nature of hallucinations in so far as the dreamer appears to see and hear what is not actually present to his senses. But it often happens that these dream-experiences are indistinct and lack sensational intensity; and in general they are without that dependence on motor activity which marks percepts. Their perceptual character is mainly due to their independence of subjective activity, — the discontinuity and abruptness of the mode of their emergence into consciousness. We are passive in relation to them in the same way in which we are passive in relation to actual objects present to the senses. Probably the hallucinations produced by suggestion in hypnotised subjects are of a similar kind.

But dream-experiences and other hallucinations have often in a greater or less degree the sensational intensity and the detailed distinctness of actual senseperceptions. They are in fact actual perceptions in all respects except that the nature of the object perceived is determined by subjective conditions rather than by external stimulation. When this is so, some kind of stimulation is generally if not always present. Among these the most essential modes of stimulation consist of certain variations in the nature and distribution of the bloodsupply within the brain, or in pathological affection of the brainsubstance. The blood may contain poisonous substances, such as alcohol, opium, ether, chloroform, and the like, which have an irritant effect on the nervous system. In sleep, owing to lowered respiration, the blood becomes charged with carbonic acid, which may have an exciting effect on the sensory centres of the brain.

Besides these general modes of stimulation, in most cases external conditions of a more special kind operate. So far as this is so, hallucinations assume the character of illusions. In an illusion, there is present some stimulation proceeding from the excitement of a senseorgan; but the object perceived differs more or less widely from that which would be perceived under normal conditions if the senseorgan were excited in the same way. Dream-Experiences are to a large extent illusions. A slight pain in the ribs makes the sleeper dream of a stab from the dagger of a threatening enemy or the bite of a dog. Contact with a cold body may give rise to the dream of a corpse. That constant stimulation of the retina which is called the retina's own light* plays a very important part in constituting dreampictures. On this subject we may quote the interesting experiences of Professor Ladd. "Almost without exception, When I am able to recall the visual images of my dream and to observe the character of the retinal field quickly enough to compare the two, the schemata of the luminous and coloured retinal phantasms afford the undoubted clue to the origin of the things just seen in my dreamlife."+ By long practice Professor Ladd has acquired the power of dropping gradually into a dreaming sleep and then suddenly awaking with his attention fixed on the comparison of his dreampictures with the experiences of light and colour due to the internal stimulation of the retina, which in his case are peculiarly brilliant and varied. "The most elaborate visual dreams may originate in intraorganic retinal excitement. Perhaps a harder problem could not be given to my experiments to solve than the following: How can one be made by such excitement to see a printed page of words clearly spread out before one in a dream? . . . But I have several times verily caught my dreaming automaton in the feat of having just performed this transformation. On waking from a dream, in which I had distinctly seen lines of printed letters forming words and sentences, and had been engaged in reading these lines by sight, I have clearly detected the character of that retinal field which had originated such an extraordinary hallucination. The minute light and dark spots which the activity of the rods and cones occasions, had arranged themselves in parallel lines extending across the retinal field."+

* See p. 151.         +Mind, n.s., vol. i. (1892), p. 301.          + Ibid., p. 302.

Pure illusions are illusions in which no element of hallucination is present. The impressions made on the senses of the observer may give rise to just the same sensations as they would normally produce, and yet the things and processes apparently perceived may not actually exist or take place. It is mainly this pure illusion, unmixed with hallucination, which is exemplified in the tricks of ventriloquists and conjurers. When a juggler swallows a sword merely in appearance, the sensory impressions made on the eye of a spectator are very much the same as if the juggler had swallowed the sword in actual fact. For this reason, pure illusions may be shared by a great number of persons simultaneously. On the other hand, collective hallucinations, though their existence is guaranteed by the Psychical Research Society, are of rare occurrence, and stand much in need of explanation.