"On Dreams" is part of Aristotle's Parva Naturalia, a collection of works on the phenomena of human life. In this treatise, he hypothesizes that we dream because our sensory organs continue to function while we sleep. He compares dreams to hallucinations—errors in our sense perception. Aristotle's view of dreams influenced philosopher Thomas Hobbes and psychologist Sigmund Freud, and it continues to fascinate readers today.