cogito ergo sum

Cogito ergo sum is Latin for "I think, therefore I am"—the philosophical proposition René Descartes offered as the one certainty that survives radical doubt.

Even if everything else might be illusion, the act of doubting proves a thinking self exists.

The phrase appears in philosophical discussion and, more casually, as shorthand for self-evident truth or overthinking: After three hours debating what to order, he finally went with pizza—cogito ergo sum.