Primer: First Principles Thinking

A first principle is a foundational assumption or truth that stands alone and cannot be further reduced or simplified.

Primer: First Principles Thinking
Photo by Nathan Duck / Unsplash

Primers are written briefs that deconstruct topics and ideas into practical steps and tools. Their purpose is practical application.

I chiefly write Primers for myself as a means to solidify my knowledge after completing an independent research project. Each primer captures and summarizes writing and concepts from multiple sources, which are cited below (not all the thinking here is original to me, as I pull out insights and ideas from multiple sources and construct them into an organized mental model).

I've been hearing the concept of "First Principles Thinking" in more and more places as an approach to enlightened problem solving. It's getting thrown around by people who want to show off their own intellectual rigor and seems to be regarded as a problem-solving elixir, so I've been curious about understanding what's behind the concept and how to employ it. This Primer captures the basics of first principle thinking. If you're interested in a deeper dive, check out the sources at the bottom.


First Principles Thinking does two things:

  1. Breaks down complicated problems into the most basic assumptions or truths. These are the fundamental, irreducible truths that underpin a problem or concept.
  2. Uses these basic assumptions and truths to identify possible solutions or approaches to the problem

What is a First Principle?

A foundational assumption or truth that stands alone and cannot be further reduced or simplified. First principles are things we know to be true.*

Think of a first principle like one Lego piece. These Lego pieces cannot be deconstructed any further, but together, they combine and recombine to create Lego sets. Here are some examples from different fields:

  • Science: scientific laws of physics, biology, chemistry
  • Math: mathematical axioms
  • Cooking: the compatibility of certain foods, temperatures, and cooking methods
  • Budget: how much individual items actually cost and then building budgets from the "bottom up"
  • Economics: the laws of supply and demand
  • Customer preferences: desire things to be easy, convenient, and inexpensive (Bezos once said that he doesn't build business models on things that change, but rather on things that don't change)
  • Mental health: The first principles of mental health are (surprisingly) physical: sleep, nutrition, exercise.
  • Sport: The rules of a game
  • Psychology: the beliefs we tell ourselves about ourselves (sometimes limiting beliefs)

*Of course, our understanding of truth is constantly changing, so we need to continuously revisit and retest our first principles. One of the long-accepted first principles of economics is that people are rational, utility-maximizing beings. The field of behavioral economics has challenged this fundamental first principle.

Example of First Principles Thinking: Cook vs. Chef

(From Tim Urban's Wait But Why)

Tim Urban writes that reasoning by first principles is different from reasoning by analogy. Reasoning by first principles deconstructs problems and then builds solutions based off the fundamental assumptions whereas reasoning by analogy searches for analogous problems and contexts and borrows from those models and approaches.

  • Reason by first Principles: The chef understands the ingredients as the basic building blocks and understands the chemistry of different foods, heat, time, etc. The chef invents the recipes based on her understanding of these basic building blocks.
  • Reason by analogy: The cook simply follows recipes. If the cook lost the recipe, she would be directionless.

Incremental thinking is often thinking by analogy, and it is the normal way we conduct our lives and our thinking. It is looking for the closest recipe, and it breeds conformity because it is set within the boundaries of the status quo, of what’s already out there. It breeds followers, not creators.

Therefore, there is a connection between first principles thinking, originality, and individual expression. Thinking by first principles leads us to ask: “What’s possible?” Individual expression comes from a place of first principles living.

Traditional thinking

First principles thinking

  • Starts with existing proof points, analogies, and examples. The initial premise is historical

  • Iteration and improvement of an existing path or set of solutions

  • Starts with the possibilities and ask: what’s the goal?

  • Define and explore a completely new path

  • Create a new recipe from the fundamental truth

The issue of form vs. function

(From James Clear)

When thinking in first principles, we are responsible for getting to the function of every principle, every building block.

What does it do? Why does it exist? What purpose does it have?

We understand the why behind something when considering first principles. For example, first principle thinking asks: what’s the best way to create light in a tent? It’s a question of function. Questions of form are: how do we make a better flashlight?

Humans tend to replicate the current form and make incremental improvements on that form, instead of investigating the underlying function. First principles thinking challenges us to set aside all forms and look squarely at the function.

  • What is the function?
  • What is the goal?
  • What is the why?

One of the traps we make when choosing form over function is that we see a thing, and immediately look at it for its traditional use-case. This is called functional fixedness. A flashlight illuminates, but it also could be used as a hammer. Therefore we need to ask the question: Does my description of the object imply a use?

Techniques to establish thinking by first principles

(from Farnam Street)

1. Socratic Questioning

Socratic questioning follows a process.

  1. Clarify your thinking and explain the origins of your ideas
  2. Why do I think this? What, exactly, do I think?
  3. Challenge assumptions
  4. How do I know this is true? What if I thought the opposite?
  5. Looking for evidence
  6. How can I back this up? What are the sources?
  7. Considering alternative perspectives
  8. What do others think? How do I know that I am correct?
  9. Examining consequences and implications
  10. What are the consequences of me being wrong?
  11. Questioning the original questions
  12. What did I originally think? What conclusions can I draw from this reasoning process?

2. The Five Whys

Ask yourself why five times to get to the bottom of something. Kids do this often, they naturally think in first principles.

3. The Elon Musk Approach

  1. Identify and define your current assumptions
  2. Breakdown the problem into its fundamental principles by asking critical questions
  3. Create new solutions from scratch

Drawbacks to First Principles Thinking

  1. It's a slow process. Not optimal for quick decisions
  2. First Principles Thinking increases risk of reinventing the wheel and not borrowing from people who have solved similar or adjacent problems. There are real advantages to reasoning by analogy. Check out Range by David Epstein for more.
  3. A faulty understanding of first principles (like limiting psychological beliefs) can lead to misguided approaches. Bad inputs lead to bad outputs.
  4. Once we arrive at first principles we believe to be true, we might be less willing to continually re-examine them for validity.