The dramatisation of subjective elements of the personality, which contributes so largely to render our dreams vivid and interesting, rests on that dissociation, or falling apart of the constituent groups of psychic centres, which is so fundamental a fact of dream life. That is to say, that the usually coherent elements of our mental life are split up, and some of them--often, it is curious to note, precisely those which are at that very moment the most prominent and poignant--are reconstituted into what seems to us an outside and objective world, of which we are the interested or the merely curious spectators, but in neither case realise that we are ourselves the origin of. An elementary source of this tendency to objectivation is to be found, it may be noted, in the automatic impulse towards symbolism by which all sorts of feelings experienced by the dreamer become transformed into concrete visible images. When objectivation is thus attained, dissociation may be said to be secondary. So far indeed as I am able to dissect the dream-process, the tendency to symbolism seems nearly always to precede the dissociation in consciousness, though it may well be that the dissociation of the mental elements is a necessary subconscious condition for the symbolism. Sensory symbolism rests on a very fundamental psychic tendency. On the abnormal side we find it in the synaesthesias which, since Galton first drew attention to them in 1883, in his _Inquiries into Human Faculty_, have become well known, and are found among between six to over twelve per cent. of people. Galton investigated chiefly those kinds of synaesthesias which he called 'number-forms' and 'colour associations.' The number-form is characteristic of those people who almost invariably think of numerals in some more or less constant form of visual imagery, the number instantaneously calling up the picture. In persons who experience colour-associations, or coloured-hearing, there is a similar instantaneous manifestation of particular colours in connection with particular sounds, the different vowel sounds, for instance, each constantly and persistently evolving a definite tint, as _a_ white, _e_ vermilion, _i_ yellow, etc., no two persons, however, having exactly the same colour scheme of sounds.[130] These phenomena are not so very rare, and, though they must be regarded as abnormal, they occur in persons who are perfectly healthy and sane. It will be seen that a synaesthesia--which may involve taste, smell, and other senses besides hearing and sight--causes an impression of one sensory order to be automatically and involuntarily linked on to an impression of another totally different order. In other words, we may say that the one impression becomes the _symbol_ of the other impression, for a symbol--which is literally a throwing together--means that two things of different orders have become so associated that one of them may be regarded as the sign and representative of the other.