cogito ergo sum

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").