Definition:
Skepticism is the philosophical attitude of questioning the possibility, certainty, or adequacy of knowledge. Skeptics argue that human beliefs and claims should be critically examined before being accepted as true. Some forms of skepticism doubt particular kinds of knowledge, while radical skepticism questions whether any knowledge is possible at all.

Thinkers:

• Pyrrho — founder of ancient skepticism.
• David Hume
• René Descartes

Example:
Questioning whether sensory experience accurately represents reality demonstrates skepticism.