René Descartes opens the Meditations on First Philosophy by announcing an ambitious program: “to demolish everything completely and start again right from the foundations.” (7:17) As the multitude of falsehoods in life has raised his suspicion of the edifice of beliefs he has inherited, Descartes aims to rebuild knowledge on a foundation that cannot be shaken. The Dream Argument is one of the central moves in the First Meditation that enables this methodic doubt. Its force lies in proving that sense-based beliefs lack the absolute certainty required to serve as an indubitable foundation. Below I explain Descartes’ line of thought in the First Meditation.
Descartes first explains that he does not need to prove all his opinions false, because undermining the basic principles of knowledge causes the rest to collapse. Here, the methodology of his radical doubt is laid: if a belief admits of any real doubt, however slight, it must be placed aside until we can find something absolutely certain. He begins from a modest epistemic insight: the senses sometimes deceive, appealing to ordinary cases like “objects which are very small or in the distance.” (7:18) Because our reasoning sometimes relies on impressions that the senses provide, even occasional deception threatens the reliability of any belief that ultimately rests on sensory input. Descartes then briefly considers the possibility of those who are insane, doubting even the most basic facts of their existence. He argues that, “Again, how could it be denied that these hands or this whole body are mine?” (7:18) He formulates, in effect, a small syllogism: madmen doubt every aspect of reality, however peculiar and unlikely; I am not mad; therefore, I should not doubt such clear truths as that “I am here, sitting by the fire, wearing a winter dressing-gown, holding this piece of paper in my hands, and so on.” (7:18)
Yet Descartes immediately distances himself from this line of reasoning, for he recognizes that even without madness, dreams can mimic such ordinary experiences with equal vividness. Thus, the reassurance that he is not insane is ultimately undermined by the introduction of the Dream Argument, which dissolves the boundary between waking certainty and dream deception.
He observes that the experiences we have in sleep are often phenomenally indistinguishable from waking experiences; when dreaming, we perceive objects, experience sensations, and form beliefs that to us in the moment seem vivid and real, such as “that I am here in my dressing-gown, sitting by the fire — when in fact I am lying undressed in bed!” (7:19) Because dreams can present themselves with such force, it is possible that any particular perceptual experience we now have might be a dream rather than a veridical perception of an external object. Descartes anticipates objections to this line. One might concede that dreams sometimes deceive about particular details but deny that they can reproduce the kind of continuity and coherence of the material world; in other words, the waking experience “happen with such distinctness” (7:19) that dreams lack. Ultimately, Descartes concludes with further observation that the fact of dreaming shows that sensory experience by itself cannot establish the difference between appearance and reality.
Having presented the skeptical force of dreams against reality, Descartes then refines his argument by considering whether dreaming nonetheless presupposes some contact with reality. He observes that even in the most extravagant dreams, where we may imagine impossible creatures or situations, the constituent elements must be drawn from real prototypes, just as painters cannot invent entirely new natures but only recombine “the limbs of different animals” into “sirens or satyrs” (7:20). Even in the case of a wholly fanciful invention, the colors used to compose it are themselves real.
Descartes extends this observation by distinguishing between complex empirical objects and the simple, universal features from which they are composed. While particular sensory judgments — “that these are my hands,” “that I am clothed in a gown” (7:18) — can be undermined by the dream hypothesis, the more general attributes remain untouched. Properties such as “the shape of extended things; the quantity, or size and number of these things; the place in which they may exist, the time through which they may endure” (7:20) are not dependent on whether one is awake or asleep. It follows that entire domains of human knowledge are differentiable: “physics, astronomy, and medicine” (7:20), insofar as they rely on empirical observation, can be suspended under the Dream Argument, while “arithmetic, geometry” and subjects “which deal only with the simplest and most general things” (7:20) survive the skeptical challenge. At this stage of the First Meditation, then, Descartes draws a provisional boundary: empirical sciences remain uncertain under the possibility of dreaming, whereas pure mathematics seems to retain its certainty.
In sum, the Dream Argument in the First Meditation works by showing that sensory experiences can present us with wholly convincing but non-veridical images, and that no present criterion guarantees we are awake rather than dreaming. Yet this distinction proves to be temporary, as Descartes closes the First Meditation by introducing the more radical hypothesis of a malicious demon. Nevertheless, the Dream Argument is crucial in preparing that only the most universal and formal structures appear capable of withstanding radical doubt.