Cartesian

"CARTESIAN, adj. Relating to Descartes, a famous philosopher, author of the celebrated dictum, Cogito ergo sum -- whereby he was pleased to suppose he demonstrated the reality of human existence. The dictum might be improved, however, thus: Cogito cogito ergo cogito sum -- 'I think that I think, therefore I think that I am;' as close an approach to certainty as any philosopher has yet made."
--Ambrose Bierce,
"The Cynic's Dictionary" (1906); republished as "The Devil's Dictionary" (1911).