The conditions of dream life, indeed, lend themselves with a peculiar facility to the formation of symbolism, that is to say, of images which, while evoked by a definite stimulus, are themselves of a totally different order from that stimulus. The very fact that we _sleep_, that is to say, that the avenues of sense which would normally supply the real image of corresponding order to the stimulus are more or less closed, renders symbolism inevitable.[146] The direct channels being thus largely choked, other allied and parallel associations come into play, and since the control of attention and apperception is diminished, such play is often unimpeded. Symbolism is the natural and inevitable result of these conditions.[147] It might still be asked why we do not in dreams more often recognise the actual source of the stimuli applied to us. If a dreamer's feet are in contact with something hot, it might seem more natural that he should think of the actual hot-water bottle, rather than of an imaginary Etna, and that, if he hears a singing in his ears, he should argue the presence of the real bird he has often heard rather than a performance of Haydn's _Creation_, which he has never heard. Here, however, we have to remember the tendency to magnification in dream imagery, a tendency which rests on the emotionality of dreams. Emotion is normally heightened in dreams. Every impression reaches sleeping consciousness through this emotional atmosphere, in an enlarged form, vaguer it may be, but more massive. The sleeping brain is thus not dealing with actual impressions--if we are justified in speaking of the impressions of waking life as 'actual'--even when actual impressions are being made upon it, but with transformed impressions. The problem before it is to find an adequate cause, not for the actual impression, but for the transformed and enlarged impression. Under these circumstances symbolism is quite inevitable. Even when the nature of an excitation is rightly perceived its quality cannot be rightly perceived. The dreamer may be able to perceive that he is being bitten, but the massive and profound impression of a bite which reaches his dreaming consciousness would not be adequately accounted for by the supposition of the real mosquito that is the cause of it; the only adequate explanation of the transformed impression received is to be found (as in a dream already narrated) in a creature as large as a lobster. This creature is the symbol of the real mosquito.[148] We have the same phenomenon under somewhat similar conditions in the intoxication of chloroform and nitrous oxide.