In his "Discourse on the Method of Rightly Conducting the Reason, and Seeking Truth in the Sciences," Descartes asserts that humans, as rational beings, should not value their dreams since they are illusionary. He states, "For, in fine, whether awake or asleep, we ought never to allow ourselves be persuaded of the truth of anything unless on the evidence of our reason." Descartes gives an example of this when he writes about the sun, and how we should not think that the sun is only as big as we perceive it.
In contrast, Freud wrote his "Interpretations of Dreams" in which he suggested that the unconscious exists and is important in understanding actions. Freud called dreams the "royal road to the unconscious," meaning that dreams show the "logic" of the unconscious mind.