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Introduction
There can be no doubt that the dream argument is one of those philosophical arguments that at first glance seem to be equaUy straightforward and simple but turn out to be most complicated and tenacious. The dream argument essentially consists of two interconnected claims: first, dreaming states are not a rehable source of the epistemic justification of behefs, and, secondly, there is no criterion to distinguish waking states from dreaming states. Whereas the first claim can be regarded as unproblematic, since we in fact do not take dreams to be an acceptable source of epistemic justification of our behefs, the second claim turns out to be the real philosophical problem. For if there is no criterion to distinguish waking states from dreaming states, we can never know whether our behefs are true, because dreaming states do not provide a rehable basis to justify them.
Descartes' Meditations represent the locus classicus of the dream argument. The reason for this is that Descartes does not consider the dream argument to be merely a philosophical idea of somewhat general interest. He rather presents a paradigmatic analysis of the dream argument. The Cartesian analysis is even the reason why the dream argument is still a controversially debated topic in contemporary episte mology and philosophy of mind. Also Fichte takes it to be a particular philosophical challenge.
To Descartes the dream argument comes in form of the so called skeptical hypothesis, i.e., an abstract description of a non-excludable skeptical possibility. According to that skeptical possibility, it could be the case that our beliefs appear to be subjectively justified though in fact they are not. By means of the skeptical hypothesis the skeptic attempts to show that we cannot know whether our behefs are true, for we are not able to rule out the skeptical hypothesis to be true. In general, the argument based on the skeptical hypothesis SH has the following structure:
(1) I do not know that not-SH.
(2) If I do not know that not-SH, I do not know that p.
Conclusion: Therefore, I do not know that p.
Thus the skeptical hypothesis argues that we cannot exclude that our epistemic situation is controlled by that very skeptical possibility. It does not claim that this in...