cogito ergo sum

pertaining to the works or ideas of French philosopher and mathematician René Descartes (1596-1650), 1650s, from Cartesius, the Latinized form of his surname (regarded as Des Cartes) + -ian. In addition to his philosophy (based on the fundamental principle cogito, ergo sum), he developed a system of coordinates for determining the positions of points on a plane.

c. 1200, cogitacioun, "thought, idea, notion, that which is thought out; act of thinking, earnest reflection," from Old French cogitacion "thought, consideration, reflection," from Latin cogitationem (nominative cogitatio), noun of action from past participle stem of cogitare "to think, reflect, consider, turn over in the mind," which is apparently a contraction of co-agitare, from assimilated form of com "together" (see co-) + agitare, here in a sense of "to turn over in the mind," literally "to put in constant motion, drive, impel," frequentative of agere "to move, drive" (from PIE root *ag- "to drive, draw out or forth, move").