Skeptical hypotheses (Descartes' evil demon, for instance) seem to rely on the

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Skeptical hypotheses (Descartes' evil demon, for instance) seem to rely on the following proposition: it is possibly that I am being systematically deceived (that all of my sensory impressions are actually infelicitous, say). My question is: is his proposition known <i>a priori</i>? or is it empirical?