First Principles

Most thinking is copying

When we face a problem we usually reason by analogy: we look at how it was done before, then make a small change. It is fast and mostly fine. It also quietly carries along every assumption the original made.

First principles thinking throws out the copy. You break the problem down to the things you know are true, things you could not argue with, and build a solution back up from only those.

A cook follows a recipe. A chef knows why heat, salt, fat and acid behave as they do, so they can invent a dish nobody has written down yet.

Take the phone apart and tap every part

Why does a phone cost ₹40,000?

Analogy says “because phones cost that much”. Let’s check what it is actually made of.

The three moves

Spot the assumptions

List everything you are taking for granted about the problem, especially the things “everyone knows”.

Dig to bedrock

Keep asking “why?” until you reach something that can’t be broken down further: physics, maths, human nature, a hard limit.

Rebuild

Design a solution using only the bedrock, ignoring how it is usually done. Then test it.

Same problem, two kinds of thinking

A school wants to cut what parents spend on textbooks. Switch between the two approaches.

Other schools negotiated a bulk discount with the publisher.
Let’s do the same and ask for 15% off.
Result: books cost ₹4,250 instead of ₹5,000. Same books, same model.

Look at both approaches

Train your eye for bedrock

The hardest part of first principles is telling a real fundamental from a habit that only looks permanent. Sort each statement. Use the buttons, or your arrow keys.

Sort all the cards

Finish the quiz

Assumption or fundamental?

Assumption0

Fundamental0

Quick check

Rebuild a problem from scratch

Pick a problem and take it through all five moves. Or bring your own.

Reach your rebuilt solution

Which problem do you want to rebuild?

What does everyone assume about this?

Select every belief you would normally take for granted. Pick at least two.

Ask “why?” until you hit bedrock

At each layer, decide: is this still a choice someone made, or is it something that can’t be argued with?

Which of these are truly fundamental?

Select only the ones that would be true whatever anyone decided. Then check your picks.

Build back up from the fundamentals

Pick the ideas that are built from bedrock. Each one shows which fundamentals it rests on. One of them is a copy in disguise.

Your own rebuilt idea (optional)

Your first-principles rebuild

Next move in real life: find the cheapest, fastest way to test one rebuilt idea before committing to it.

Reason up from what is undeniably true instead of across from what others have done.

What do I know for certain?

Reach for it when

Everyone says something can’t be done cheaper, faster or differently.
A whole industry or team does things one way and nobody remembers why.
Standard advice has stopped working for you.
You are designing something genuinely new.
A cost or limit seems “fixed” and it is blocking you.

Put it down when

The decision is small and everyday. Copying is faster and good enough.
You don’t yet know the field well enough to tell bedrock from habit. Learn first.
The clock is ticking and a proven playbook already works.

The framework

Define the problemWrite it in one line, and note the current way it is solved.
List the assumptionsEverything you take for granted, especially “that’s just how it’s done”.
Ask why, layer by layerKeep going until you reach physics, maths, evidence or human nature.
Keep only the fundamentalsCross out anything that is really a choice someone made.
Rebuild from zeroCombine the fundamentals into new solutions, ignoring convention.
Test cheaplyPick the riskiest new idea and find the smallest experiment to check it.

Rules of thumb

If you can’t explain why something is true, treat it as an assumption.
Physics, maths and well-tested evidence count as bedrock. “Everyone does it” doesn’t.
Use analogy for speed, first principles for breakthroughs.
People and their habits are real constraints too, not just materials.

Questions to ask yourself

What am I assuming that I have never checked?
Why is it done this way, and is that reason still true?
If I started from zero today, what would I build?
What is the raw cost or the hard limit here?

Common traps

Stopping at the first “why” and calling it bedrock.
Reinventing everything. Some conventions exist for very good reasons.
Mistaking your field’s habits for laws of nature.
Falling in love with the rebuilt idea and skipping the test.

All mental models Next: Probabilistic Thinking