Full Idea
I will suppose an evil genius, supremely powerful and clever, who has directed his entire effort at deceiving me. I will regard all external things as devilish hoaxes, and myself as not possessed of a body or senses, but falsely believing these things.
Gist of Idea
To achieve full scepticism, I imagine a devil who deceives me about the external world and my own body and senses
Source
René Descartes (Meditations [1641], §1.22)
Book Reference
Descartes,René: 'Discourse on Method/The Meditations', ed/tr. Sutcliffe,F.E. [Penguin 1968], p.100