Content
Aristotle explains that during sleep there is an absence of external sensory stimulation. While sleeping with our eyes closed, the eyes are unable to see, and so in this respect we perceive nothing while asleep. He compares hallucinations to dreams, saying "...the faculty by which, in waking hours, we are subject to illusion when affected by disease, is identical with that which produces illusory effects in sleep." When awake and perceiving, to see or hear something incorrectly only occurs when one actually sees or hears something, thinking it to be something else. But in sleep, if it is still true that one does not see, hear, or experience sense perception in the normal way, then the faculty of sense, he reasons, must be affected in some different way.
Ultimately, Aristotle concludes that dreaming is due to residual movements of the sensory organs. Some dreams, he says, may even be caused by indigestion:
We must suppose that, like the little eddies which are formed in rivers, so the movements are each a continuous process, often remaining like what they were when first started, but often, too, broken, into other forms by collisions with obstacles. This gives the reason why no dreams occur in sleep after meals, or to sleepers who are extremely young, e.g., to infants. The movement in such cases is excessive, owing to the heat generated from the food. Hence, just as in a liquid, if one vehemently disturbs it, sometimes no reflected image appears, while at other times one appears, indeed, but utterly distorted, so as to seem quite unlike its original; while, when once the motion has ceased, the reflected images are clear and plain; in the same manner during sleep the images, or residuary images are clear and plain; in the same manner during sleep the images, or residuary movements, which are based upon the sensory impressions, become sometimes quite obliterated by the above described motion when too violent; while at other times the sights are indeed seen, but confused and weird, and the dreams are incoherent, like those of persons who are atrabilious, or feverish, or intoxicated with wine. For all such affections, being spirituous, cause much commotion and disturbance.
Aristotle also describes the phenomenon of lucid dreaming, whereby the dreamer becomes aware that he is dreaming.