On the one hand, it would appear that most, perhaps, indeed, all dreams that are sufficiently vivid to be clearly remembered on awakening, have received an initial stimulus from some external, or at all events, peripheral source.[56] There is something unusual or uncomfortable in the sleeper's position, or he has been subjected to some slight unusual strain which has modified his nervous condition, or there has been some deviation from his usual diet, or a physiological stress of some kind is making itself felt within him--careful self-questioning constantly reveals the actual or probable existence of some external or certainly peripheral stimulus of this kind. So that we seem entitled to say that in all dreams there is probably a presentative element. On the other hand, an even more cursory investigation of our dream life suffices to show that in every dream there is also a representative element. No dream can be said to be strictly and literally presentative. If, when I am seemingly asleep, a person speaks to me, and I become conscious that he is present and speaking, I am not entitled to say that I 'dream' it. A consciousness which perceives facts in the same way as they may be perceived by waking consciousness is not a dreaming consciousness. So that there are, in the literal sense, no presentative dreams. What happens is that the stimulus, instead of being presented directly to consciousness, and recognised for what it is to waking consciousness, serves to arouse old memories and ideas which dream consciousness accepts as a reasonable explanation of the external or peripheral stimulus. The stimulus may be said to be, in a sense, the cause of the dream, but the dream itself remains central, and as truly a combined picture of mental images as though there were no known peripheral stimulus at all. Thus, while it is true that the division of dreams into two classes corresponds to a recognisable distinction, it is yet a superficial and unimportant distinction. It is likely that all dreams have a peripheral or presentative element, and certain that they all have a central or representative element. This will become clearer if we now proceed to discuss those dreams which have, demonstrably, their exciting cause in some external or internal organic stimulus.