[Footnote 52: Edmund Parish, _Hallucinations and Illusions: A Study of the Fallacies of Perception_ (Contemporary Science Series), 1897. It is significant to observe that in hysteria, which may be regarded as presenting a condition somewhat analogous to sleep, dissociation also occurs. 'Hysteria,' says Janet (_The Major Symptoms of Hysteria_, 1907, p. 332), one of the greatest authorities, 'is a form of mental depression characterised by the retraction of the field of personal consciousness and a tendency to the dissociation and emancipation of the system of ideas and functions that constitute personality.'] [Footnote 53: The theories of attention are lucidly and concisely set forth by Nayrac, 'Le Processus et le Mécanisme de l'Attention,' _Revue Scientifique_, 7th April 1906.] [Footnote 54: G. F. Stout, _Analytic Psychology_, vol. ii. p. 112. In the _Dictionary of Philosophy and Psychology_, again, Stout and Baldwin define apperception as 'the process of attention in so far as it involves interaction between the presentation of the object attended to, on the one hand, and the total preceding conscious content, together with pre-formed mental dispositions, on the other hand.'] [Footnote 55: A very similar state of things occurs in some forms of insanity, especially in the less profound states of mental confusion, when, as Bolton remarks ('Amentia and Dementia,' _Journal of Mental Science_, July 1906, p. 445), we find 'certain associated remnants of former experience combined into a sequence according to the normal laws of mental association.'] CHAPTER IV THE SENSES IN DREAMS All Dreams probably contain both Presentative and Representative Elements--The Influence of Tactile Sensations on Dreams--Dreams excited by Auditory Stimuli--Dreams aroused by Odours and Tastes--The Influence of Visual Stimuli--Difficulty of distinguishing between Actual and Imagined Sensory Excitations--The Influence of Internal Visceral Stimuli on Dreaming--Erotic Dreams--Vesical Dreams--Cardiac Dreams and their Symbolism--Prodromic Dreams--Prophetic Dreams. At the outset I adopted provisionally the usual classification of dreams into two classes: the peripheral or presentative group, excited by a stimulus from without, and the central or representative group, having its elements in memories. If, however, we look carefully at the matter, in the light of the experiences which we have encountered, it will be found that this classification, however superficially convenient it may be, fails to correspond to any radical duality of dream phenomena. When we closely question our dream experiences, it ceases to be clear that they really fall into two groups at all.