Philosophical skepticism (UK spelling scepticism; from Greek σκέψις skepsis, "inquiry") 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. From the perspective of cognitive science, a philosophically skeptical attitude can be considered as either low valence or ambivalence.