The philosopher best known for his scepticism is Rene Descartes. Descartes’ main legacy to philosophy was doubt. Ironically, Descartes himself was not a sceptic; though he proposed various sceptical arguments that have subsequently proved difficult to refute, Descartes offered responses to each of them. These responses, however, have convinced few; it is his sceptical arguments that have had the greatest impact on philosophy.
Descartes’ doubt, set out in his Meditations on First Philosophy, comes in three waves. In the first wave of doubt, Descartes advances the argument from error, arguing that as our senses have led us astray before we should not trust them in future. In the second wave, he advances the argument from dreaming, arguing that all of our experiences are as consistent with the hypothesis that we are dreaming as they are with the hypothesis that we are awake, and so we cannot know which hypothesis is true. In the third wave, he advances the argument from deception, invoking the idea of an evil demon constantly deceiving us as a troubling hypothesis that cannot easily be dismissed.