The dream argument (In René Descartes' Meditation and in Philosophy in General) is the assertion that the act of dreaming provides intuitive evidence such that it is indistinguishable from that which our senses provide to us in the waking state, and that, for this reason, we cannot fully trust the senses we use to distinguish reality from illusion . Consequently, any experience coming from our senses should at least be carefully examined and rigorously tested to determine if it really refers to "reality".
Descartes uses the argument of the dream in his Meditations on First Philosophy (Meditation 1) to show the uncertain nature of the information given by the senses. The argument thus takes place in a series of thought experiments: optical illusions, then madness and evil genius. Descartes emphasizes the realistic nature of the dream, and the difficulty of distinguishing it from sensation in the waking state: "How many times have I dreamed, at night, that I was in this place, that I was dressed, that I was near the fire, although I was quite naked in my bed? But this argument, if it is relevant to questioning sensible knowledge, which is why Descartes is looking for others, to end up with the evil genius. Descartes ends up giving himself the refutation of this argument of the dream: "our memory can never bind and join our dreams to each other and with all the continuation of our life, as it is wont to join things that happen to us while awake". It is the continuity that will make it possible to distinguish the dream from the day before.
See also: Descartes' Wax Argument Explained