Dream argument

The dream argument uses the possibility or character of dreaming to challenge claims that waking experience provides secure knowledge of the external world.

Dream skepticism: The skeptical view that one may be dreaming and therefore mistaken about the apparent external world. It states the skeptical conclusion the argument uses dreams to motivate.
René Descartes: A seventeenth-century French philosopher and mathematician who developed methodological skepticism and Cartesian dualism. His Meditations gives the argument its canonical philosophical formulation.
Evil demon: Descartes's hypothetical deceiver with the power to make a person systematically mistaken about experience. Unlike ordinary dreaming, the demon hypothesis threatens even apparently stable reasoning.
Cogito ergo sum: Descartes's conclusion that the act of thinking establishes the existence of the thinker. Even if experience is dreamlike, Descartes argues that doubting confirms a thinking subject.
Argument from illusion: An argument that perceptual errors suggest perception does not always reveal objects exactly as they are. Dream experiences extend ordinary perceptual error into a challenge to waking experience as a whole.
Meditations on First Philosophy: Descartes's 1641 work examining certainty, God, the mind, and the material world. The First Meditation develops the dream argument as part of a search for certainty.
Brain in a vat: A thought experiment in which a brain receives artificial experiences that mimic life in a world. It presents a more technologically specified alternative to dreaming as a source of error.
Foundationalism: The theory that knowledge rests on basic beliefs that do not depend on inferential support from other beliefs. The argument pressures foundational projects to identify beliefs secure against dream-based doubt.
Skeptical hypothesis: A scenario designed to make ordinary beliefs compatible with radical error about the world. The dream possibility functions as a skeptical hypothesis about present experience.
Augustine of Hippo: A late antique Christian theologian and philosopher whose writings address knowledge, memory, and the self. Augustine discusses the difficulty of distinguishing dreams from waking life before Descartes.

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