Lesson · 4 min Stretch
Test Your Certainty
In one line Ask of each belief whether any reason to doubt it could be given, and rank them by the answer.
Descartes wanted a firm footing for knowledge, so he tested his beliefs by trying to doubt them. A belief that survives a serious attempt to find a reason against it earns more trust than one that simply feels strong.
Not all beliefs fare alike. “The shop closes at five” rests on a sign I read last year, and many things could have changed. “Two plus three is five” is harder to doubt, since no change in the world would alter it. Sorting beliefs by how hard they are to doubt gives a rough map of where to lean.
Doing this does not mean doubting everything. It means knowing which beliefs can bear weight, and checking the shaky ones before acting on them.
The shop hours
Tariq plans to meet a client at a shop that, he is sure, closes at five. He asks what could make him wrong and lists a holiday, a new schedule and an old sign.
He checks the shop’s website and finds it now closes at four. The belief felt strong but was easy to doubt. His belief that four o’clock comes before five needed no check.
Try this today List three things you feel sure of, and for each write what could make it wrong.
Cite or share this lesson
A plain reference you can paste into notes or an essay:
Quest2Reason. “Test Your Certainty.” https://quest2reason.com/learn/lesson/test-your-certainty. Accessed 13 November 2026.
Sources
- Stanford Encyclopedia of Philosophy, “Descartes’ Epistemology” — Covers his method of doubt and his test of clear and distinct ideas.
- René Descartes, Meditations on First Philosophy (1641), translated by John Veitch — Descartes’ own text on doubt and certainty, in English.