Skepticism is the attitude of doubting claims to knowledge. It asks whether we can know anything at all. Ancient skepticism, associated with Pyrrho and Sextus Empiricus, held that we should suspend judgment about everything, because for every argument there is an equal and opposite argument. The result is tranquility, not despair. Modern skepticism, associated with Descartes, used doubt as a method. Descartes doubted everything until he reached the certainty of his own existence. He then tried to rebuild knowledge on that foundation.

Skepticism comes in many forms. Global skepticism doubts all knowledge. Local skepticism doubts knowledge in a specific domain, such as morality or the external world. Cartesian skepticism doubts the existence of the external world, using the dream argument and the evil demon hypothesis. Humean skepticism doubts causation and induction. Skepticism is not the same as cynicism, which distrusts motives, or nihilism, which denies value. It is a methodological stance, a way of testing claims. Some philosophers argue that skepticism is self-refuting. If you know that you cannot know anything, then you know something. Others argue that skepticism is a useful goad, forcing us to examine our assumptions. The debate continues.

Forms of skepticism

• Global. We cannot know anything.
• Local. We cannot know things in a specific domain.
• Cartesian. We cannot know the external world exists.
• Humean. We cannot know causation or induction.

Skepticism is not the same as doubt. Doubt is a feeling. Skepticism is a position about the possibility of knowledge.