What did Descartes contribute to critical thinking with his "methodical doubt" and "cogito ergo sum"?

He made doubt a deliberate method: doubt everything that can possibly be doubted, to find what remains certain — and what remains is "I think, therefore I am."

Descartes turned doubt from a vice into a tool. Methodical doubt means systematically suspending belief in anything that could conceivably be false — the senses (they deceive), even mathematics (an evil deceiver might trick you) — to see whether any foundation survives. What survives is the act of doubting itself: even to doubt, you must be thinking, and to think you must exist. Hence "cogito ergo sum" — I think, therefore I am.

The critical-thinking lesson isn't the specific conclusion but the move: doubt is not the enemy of reason but its instrument. Used methodically, doubt clears away inherited certainties so that whatever you end up believing has actually been tested.