1. an attitude of questioning, disbelief, or doubt.

2. in philosophy, the position that certainty in knowledge can never be achieved. David Hume made skepticism a cornerstone of his system and provoked much later discussion when he taught that sensory experience provides no sure basis for knowledge of the external world and that nothing can be proved by observation. Causation, for example, is only an inference that relates two observed events, and one has no knowledge that this relationship will apply in similar cases; it is a generalization that could be proved wrong by a different result. In modern philosophy, postmodernism, poststructuralism, and deconstruction are essentially systems of skepticism. —skeptic n. —skeptical adj.