Aristotle wrote two short pieces on dreams and their meaning: On Dreams and On Prophesying on Dreams. In the first he presents arguments for the origin of dreams and their relationship to other aspects of human existence. In the second he analyzes whether dreams can foreshadow the future.
Aristotle approaches his subject as a scientist, looking for a practical explanation for an experience men found mysterious and frightful. Given that the only ways to acquire knowledge are through sense perception or intelligence, and that our senses are not operating during sleep, he asserts that the senses cannot contribute to dreaming. That means dreams must originate in our intellect. Of course the experiences in dreams are still based on sense perception (we see things when we dream), but that experience is disconnected from reality.
Residual perceptions are common in everyday life. If you look at the sun briefly and then look away, the image of the sun remains in your eye for a while. In the same way, Aristotle asserts that when the external object of a perception has been removed, there still remains an impression which itself is an object of perception. While we sleep, these impressions are re-created in dreams.
Sensory impressions present themselves when an individual is awake and asleep, but the senses work with the brain during the day to keep reality in perspective. At night there is no balance between impressions because the senses are not available, so dreams produce wild and obscure sensations.
Aristotle describes the dream sequence as “little eddies in a river forming and breaking into other forms by colliding with obstacles.” He believed there could be no dreaming immediately after a meal because the heat caused by digestion disturbed the flow of phantasms in the brain. Similarly, dreams that occur during a fever or intoxication reflect a disturbed state of the body in their weirdness of form and character.
In On Prophesying on Dreams, Aristotle contemplates what he calls the divination that takes place during sleep and whether it caused by dreams. Since there is no known cause of this divination it is normal to be skeptical about it. It cannot come from a god because these experiences occur in common people and god does not communicate to them. So is it merely that dreams act as causes or tokens of divination? Indeed, it may be that some of these representations are the causes of actions cognate to them. We may think and plan some activity during the day whose significance causes a vivid dream at night. In this case the activity has paved the way for the dream. But the converse is also true, because thoughts which occur first in sleep may be the starting points of something that occurs when we are awake.
Aristotle believes that most “prophetic” dreams are coincidences based on the fact that the dreamer has no real participation in the story of the dream. We often mention things during conversation that later come to pass and this same phenomena occurs in dreams. Because the engine of both wakefulness and dreams is the brain, we understand why this has to be so. Again, because god does not communicate to the common people, their visions must be a random result of their physical temperament – “excitable and garrulous”. The common people have chance experiences where visions play a part in their slumber, like the gambler who plays even and odd.
Prophetic dreams are caused by the condition of sleep; the fact that there is less to disturb the body than during the day. There is no wind at night to disturb the senses and compete with the visions of our dreams.
To Aristotle, the most skillful interpreter of dreams is the man who is able to observe resemblances in them. That is he can make sense out the forms in disturbed water; to put the pieces together which, to the common man, can only be seen when the water is calm.