The Evil Demon Hypothesis

Descartes imagined an all-powerful malicious being feeding him completely false sensory experiences. Every sight, sound, and touch might be an elaborate deception. The evil demon hypothesis pushed doubt to its logical extreme: what if everything you perceive is manufactured?

This thought experiment, from his 1641 Meditations, is the ancestor of the brain-in-a-vat scenario, The Matrix, and the simulation hypothesis.

Yet the cogito survives even the demon. The demon can deceive you about everything except the fact that you exist as a thinking being, because deception requires someone to be deceived.