In dreams we see much, though we but seldom understand; we are powerfully agitated by our sensations, images follow each other, without the least intervention of the mind, either to compare or reconcile them. We have sensations, then, but no ideas, the latter being comparisons of the former; so dreams must reside solely in the internal material sense; and as the mind does not produce them, they must form a part of that animal reminiscence, of which we have already treated. Memory, on the contrary, cannot exist without the idea of time, without a comparison of ideas, and as these extend not to dreams, it seems to be obvious that they can neither be a consequence nor an effect, nor a proof of memory. But though it should be maintained that to some dreams ideas certainly belong; and as a proof of it, those people be quoted who walk, speak, and converse connectedly while asleep; still it would be sufficient for my argument, that dreams may be produced by the renovation of sensations alone, for in consequence thereof the dreams of animals must be merely of this species, and such dreams, far from supposing memory, indicate nothing but a material reminiscence.
By no means am I inclined to believe, that persons who walk and converse while asleep are in reality occupied with ideas. In all such actions the mind seems to have no concern. Sleep-walkers go about, return and act, without reflection or knowledge of their situation or danger; alone are their animal faculties exercised, and even of these some remain unemployed; and while in this state, a sleep-walker is of course more stupid than an idiot. As to persons who speak while asleep, they never say any thing new. An answer to certain common questions, a repetition of a few familiar expressions, may be produced, independent of the principle of thought or action of the mind. Why should we not speak without thought when asleep, since when most awake, and under the influence of passion, man utters numberless things without reflection.
As to the occasional cause of dreams, by which former sensations are renewed without being excited by present objects, it is to be observed, that we never dream when our sleep is sound: every thing is then in a state of inaction, and we sleep both outwardly and inwardly. The internal sense, however, falls asleep the last, and awakes the first, because it is more active, and more easily agitated, than the external senses. It is when our sleep is less sound that we experience illusive dreams, and former sensations, those especially which require not reflection, are renewed. The internal sense being unoccupied by actual sensations from the inaction of the external senses, exercises itself upon its past sensations. Of these the most strong appear the most often; and the more they are strong, the more the situations are extravagant; and for this reason it is, that almost all dreams either terrify or charm us.