Most of the time, we reason by analogy. Someone asks us a hard question, and we answer by comparing it to something similar we’ve already seen — “this is basically like that.” It’s fast, and most days it works fine. But analogy also quietly caps how far your thinking can go, because you’re always bound by what already exists.
There is an older, sturdier way of thinking, and its clearest modern description comes from philosopher René Descartes in his book “Discourse on the Method.” Descartes wrote it in 1637, trying to solve a much bigger problem than productivity — he was trying to figure out how to trust any conclusion at all. His answer became the foundation of what we now call first principles thinking.
What Descartes Actually Proposed
Descartes laid out four rules for reasoning that still hold up almost four hundred years later:
1. Accept nothing as true unless you can verify it yourself, clearly and without doubt.
2. Break every difficult problem into as many smaller parts as necessary to solve it properly.
3. Order your thoughts, starting from the simplest, most certain truths, and build upward toward the more complex.
4. Review the whole chain of reasoning often enough to be sure nothing was left out.
Strip away the 17th-century language, and this is exactly what “first principles thinking” means today: instead of borrowing someone else’s conclusion, you break a problem down to the truths you’re actually certain of, and rebuild your answer from there.
Why This Is Harder Than It Sounds
Reasoning by analogy is comfortable because it borrows credibility from things that already worked. First principles reasoning strips that away. You’re forced to ask “how do I actually know this is true?” about assumptions you never questioned before — and that process is slower, and often uncomfortable, because it exposes how much of our thinking is inherited rather than verified.
Putting It to Work
Step 1 — State the assumption you’re currently relying on.
Write down the belief your current plan depends on, in one plain sentence.
Step 2 — Ask what you actually know to be true, independent of that assumption.
Not what’s commonly believed. Not what’s convenient. What can you verify.
Step 3 — Rebuild from there, one verified step at a time.
Descartes’s method isn’t about ignoring existing knowledge — it’s about refusing to stack new conclusions on top of unexamined ones.
Step 4 — Check the whole chain before you commit.
Walk back through your reasoning once more. If any link depends on “well, that’s just how it’s usually done,” go back and rebuild that link too.
Why It Matters
First principles thinking is slower than reasoning by analogy, and that’s exactly why it works when analogy fails — in genuinely new situations, where nothing comparable exists yet. Descartes wasn’t trying to make people faster thinkers. He was trying to make sure that when we finally arrived somewhere, we could trust how we got there.
