In 1641, Descartes sat alone in a room and asked himself: what can I know for certain?

Not "what do I believe?" Not "what seems true?" But: what survives total doubt?

He stripped away everything — his senses, his memories, even mathematics. And he landed on one unshakeable truth: the act of doubting itself proves something exists to do the doubting. Cogito ergo sum.

Most people treat this as a fun philosophy fact. But it's actually a devastatingly practical tool.

Here's why it matters now:

Every decision you make is built on assumptions. Your career choice, your relationships, your politics — all of it sits on a foundation of beliefs you've probably never examined.

Descartes' method — radical doubt — is basically a stress test for your worldview. You take each belief and ask: "If I couldn't rely on the source of this belief, would I still hold it?"

Try it with one belief you hold strongly. Just one. You might be surprised how many of your convictions are inherited rather than earned.

That's the kind of thinking I explore every week in First Principles — one philosophical idea, made genuinely useful. No academic jargon. No fluff. Just sharp thinking you can actually apply.

The unexamined life, as Socrates said, is not worth living. Start examining.