Gratuitous Descartes Joke

René Descartes (1596-1650) by Frans Hals.

The famous assertion by René Descartes, "Cogito ergo sum," ("I think therefore I am") has often been challenged on the grounds that it starts with "I."

If Descartes attempts to prove his own existence, he ruins his argument by presupposing his own proposition.

He puts de cart before the horse.