First principles reasoning: upgrade your mental software and change your life

First principles reasoning tune the engines of progress more than any other mental tool. It’s a process used heavily in physics and recently popularised by innovators like Elon Musk and Reed Hastings. Now it’s so popular that it’s becoming misused. This post is dedicated to nailing down how first principles reasoning works, and to make sure you use it appropriately.

What are first principles?

First principles are basic propositions that can’t be deduced from other conclusions or assumptions. Why go through this process? Life is full of incorrect conclusions that other people make. So assembling the basic observations that don’t borrow from other people’s conclusions helps us understand reality more accurately.

Why is first principles reasoning useful?

You become able to draw completely novel conclusions that may be more useful than the status quo. So if you’re stretching for big goals that produce exponential returns, first principles reasoning is the nitrous you put into your car to take you there. And here’s why.

First principles adds nitrous to your engine, so you accelerate towards your goals.

Most reasoning we do in day-to-day life is “reasoning by analogy” meaning it’s knowledge passed down from other people. Or the reasoning is “analogous”, where you borrow someone else’s process from a similar situation.

By borrowing other people’s reasoning, you can at best achieve the same returns as them. You are the cook reading recipes instead of the chef, who takes the raw ingredients and builds a recipe from scratch. The chef builds completely new recipes, which can be much better than the old ones. Here’s how to become a chef.

How to reason from first principles

First principles reasoning is the process where you distill facts to their core truths, and then reason up from there. 

Here are the steps:

  1. Understand as many facts as you can about a situation.
  2. Filter the facts so that you only have the ones that don’t rely on other people’s conclusions. 
  3. Examine the likelihood that the remaining facts are true. 
  4. Determine which facts are most important to the problem at hand. 
  5. Use reasoning to draw a new conclusion, based on the facts. Reasoning is a huge topic by itself, but a good way to think of reasoning is David Deutch’s shorthand he uses in the beginning of infinity; creating, combining, altering and criticizing ideas.

How do you learn from first principles?

You can learn using first principles by breaking down a subject into its basic components, then building a knowledge tree. A knowledge tree starts with basic observational truths that are a sturdy foundation (the trunk). Then you can start to build out the branches and the leaves.

For instance, learning computer science could look something like this:

  1. Learn the physics of transistors, logic gates, and other parts of computer architecture.
  2. Learn how information flows through a computer, and learn how information is transformed and organised (data structures and algorithms).
  3. Learn how compilers and interpreters work
  4. Learn boolean logic.
  5. Then learn how programming languages work, and how they compile.

And if you have enough time…

  1. Compare your conclusions with the status-quo
  2. See if you can get outside criticism from others to shoot down your conclusions
  3. Test your conclusions 

The limitations of first principles

The computational limits of first principles

First principles reasoning takes a lot of computational power because you need to reason from scratch. It’s like having a programmer write a program from scratch versus copying and pasting someone else’s code. 

It would be great if we could reason from first principles for every decision, but that’s simply not practical. You’d simply start to run out of time. It’s a big reason why the wisdom of the crowds is also powerful, drawing on the wisdom of others can be a much more efficient route.

There’s many professional and academic systems that obey these computational limits for good reason, such as common law. It would take way too much time and brain power to re-examine the fundamentals of law with every case, so previous opinions and decisions are drawn on to make future decisions.

Where do you draw the line on what is a “first principle”?

When you’re doing the process of finding your first principles on a topic, it’s not clear when you should stop. Take the process of learning to become a doctor. Here’s how they might think about first principles:

“Alright, let’s start with the first principles of biology. Okay, so there’s DNA, amino acids, proteins, etc. But wait, all of these concepts are really the branches on other physics and chemistry trees? Does that mean I should be really learning the right chemistry concepts that build up to amino acids? And even when I get there, there’s more layers underneath I should know”

Judgement is the only way around this problem. If we look at DNA for instance and try to understand it, it’s not something that varies across all living things. So that could be a good reason to establish it as a first principle.

Doctors run into the same issue mentioned before, which is that there is probably way too much information to learn if you combine modern medicine with biology, chemistry, and physics. So the end result is a lot of rote memorization of procedures and information. Here’s a good criticism from twitter:

“…it’s especially grating since my mental model of doctors was always like “flesh engineers” which implies that they should be fairly innovative, STEM brained first principles thinkers, etc. but no it’s more like they’re technicians that have memorized an object model graph”

– Roon (@tszzl on twitter)

First principles are easy to overuse because of their reach

First principles have a far and wide reach, so it’s tempting to use them in every part of your life. But there comes a point where you become delusional on what’s really possible when you use it as a tool too much. 

If you’re starting a department from scratch for instance, you may be tempted to build all of your own management tools and procedures from scratch using “first principles”. But you might just be inexperienced and using the concept to excuse yourself from trying best practices. 

Some examples of first principles reasoning in action

How Einstein developed relativity: Einstein went back to the assumptions about how distance and speed work. He concluded that when it comes to understanding absolute speed, distance and time are actually the variables that change. Which led to a lot of really wacky, unintuitive things about how the universe works, such as time dilation. 

SpaceX builds the world’s cheapest orbital rockets: before the turn of the millennia, it was common knowledge that rockets could only be used once. But if we do the right first principles exercise, from a physics point of view, there’s nothing stopping one from landing a rocket propulsively. The cost of a rocket is mainly the raw materials, whereas the fuel represents a fraction of the cost. So making rockets reflyable would dramatically reduce material costs. After more than 15 years, the team at SpaceX eventually figured out how to land the first stage of a rocket, and re-fuel it.

Elon Musk’s definition of first principles

Here is a quote of Elon Musk talking about first principles in action: 

“People would say, ‘Historically it’s cost $600 per kilowatt-hour, and so it’s not going to be much better than that in the future.’ And you say, ‘No, what are the batteries made of?’ First principles means you say, ‘Okay, what are the material constituents of the batteries?’ You just have to think of clever ways to take those materials and combine them into the shape of a battery cell, and you can have batteries that are much, much cheaper than anyone realizes.”

– Elon Musk

Where should first principles be used as a starting point?

The best place to start is to examine your own thoughts. Your brain has software downloaded without analysis by other people. It’s important to form a chain of “why’s” to tunnel down on your uninspected beliefs. You’ve inherited legacy software from parents and close friends which you’ll eventually be able to toss out.

You can even question deeply-held parts you believe about yourself. Maybe you think you’re an anxious person. But why do you believe that? If you go back far enough, maybe you’ll discover you concluded that because under certain conditions, you’d get really anxious. But it didn’t fully explain what was going on. So you’re able to unwind things about yourself that were set in stone.


Use first principles as a tool to get exponential returns in your life. Beware of the limitations that they pose however. When you have a hammer, everything is a nail, so it’s important to realize that first principles reasoning is one tool in your kit to construct your life. Many history’s greatest thinkers used this mental model in some form or another, whether knowing the concept or not, so use it to join the select few. 

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