I think; therefore I am

Cogitō ergo sum

René Descartes Méditations sur la philosophie première (1641).

L. cogitō
To think, ponder, consider, plan

je pense donc je suis

Fr. penser
To think, reckon; guess, imagine; reflect; realize, figure; wonder
I wonder (pense) what Descartes meant. I have no idea if Descartes translated his own work into Latin, but the original is in French, which is significant because it was one of the first, if not the first, philosophical treatise written in the vernacular. The English saying, “I think, therefore I am,” comes from the Latin translation (1644). To me, I am certain of my existence because I am aware of my own existence. What I think can be erroneous, as it usually is, but being aware of myself is without question. The difference in meaning between I think, cogitō, and pense is notable. The connotations are different. I believe that Descartes really meant wonder, realize, even imagine. These meanings are not the strenuous mental exercise that is thinking.

People who dismiss Descartes as nonsense have either not read the Meditations, or they have not understood him.

In the context of Philosophy, reason is not a natural activity but one that takes a good deal of study and practice. Rhetoric is the academic discipline that applies abstract reasoning to practical matters. In every day speech, rhetoric usually refers to obfuscation, but in actually it is anything but. Rhetoric conveys meaning as close to perfectly as is possible, given the clumsy nature of language.

Sometime in the future, I plan to convey my thoughts on how another French philosopher, Jean-Paul Sartre, in his treatise Being and Nothingness (also written in French, 1943), builds on Descartes.