The Scientific Literature Dealing with the Problems of Dreams
Freud introduces The Interpretation of Dreams by stating that he's about to establish proof "that there is a psychological technique which makes it possible to interpret dreams" (1.1.1).
As a starting point, Freud begins by reviewing previous theories of dream interpretation.
Freud argues that prehistoric views of dreaming are echoed in the views of the people of classical antiquity (that's ancient Greece and Rome, basically). For these people, dreams had supernatural and metaphysical significance: "they were revelations from gods and daemons" (1.1.4).
Skipping ahead to Aristotle, Freud notes that for the Greek philosopher, dreams were part of "the mental activity of the sleeper" (1.1.5).
Freud compares Aristotle's views with those of others from antiquity, discussing theories in which dreams are understood as prophecies and revelations from higher powers.
Under these circumstances, Freud says, the interpretation of dreams was an attempt to understand what the gods were saying—and what the future would hold.
After these introductory comments on dreams in antiquity, Freud moves ahead with a review of scientific theories on dreaming.