MacDonald says that "Descartes' proof that the state of waking cannot be distinguished from sleep depends upon these facts: (a) that dreams are remembered, and (b) that what is so remembered resembles an object or combination of objects perceived when awake." But MacDonald argues that the criteria of "real" and "illusory" physical objects which apply to the perceptions of waking life cannot be applied to illusions.

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MacDonald's statement is a misinterpretation of Descartes' argument. Descartes does not claim that the state of waking cannot be distinguished from sleep based on the memory of dreams and their resemblance to waking perceptions. Instead, Descartes argues that because we can have vivid dreams that are indistinguishable from waking experiences, we cannot rely solely on our sensory perceptions to distinguish between being awake and dreaming. MacDonald's argument about the criteria of "real" and "illusory" objects not applying to illusions in the same way as they do to waking perceptions is not directly related to Descartes' primary concern, which is the reliability of our senses to discern the nature of our existence and the external world.

MacDonald Says That "Descartes' Proof That the State of Waking