Philosophical skepticism is both a philosophical school of thought and a method that crosses disciplines and cultures.
It is generally agreed that knowledge requires justification. It is not enough to have a true belief: one must also have good reasons for that belief. Skeptics claim that it is not possible to have an adequate justification.
Skepticism is not a single position but covers a range of different positions. In the ancient world there were two main skeptical traditions. Academic skepticism took the dogmatic position that knowledge was not possible; Pyrrhonian skeptics refused to take a dogmatic position on any issue—including skepticism. Radical skepticism ends in the paradoxical claim that one cannot know anything—including that one cannot know about knowing anything.
Skepticism can be classified according to its scope. Local skepticism involves being skeptical about particular areas of knowledge, e.g. moral skepticism, skepticism about the external world, or skepticism about other minds, whereas global skepticism is skeptical about the possibility of any knowledge at all.
UK spelling scepticism; from Greek σκέψις skepsis, "inquiry"