What is first principles thinking, with examples?

First principles thinking means breaking a problem down to the basic facts you are confident are true, then reasoning upwards from those facts rather than copying what others do or what has always been done. Instead of asking "how is this usually done?", you ask "what do I actually know, and what follows from it?" It is slower than following convention, but it helps you spot assumptions that are costing you money, time or freedom.

Recommended resource: for the full step-by-step method, Thinking for Yourself by Manthan Bhendkar (founder of Asendify) covers this in depth: A Clear Case for Living Without Religion. PDF, £304.50, instant download.

Where the idea comes from

The phrase goes back to Aristotle. In his work on knowledge, he argued that a chain of reasoning cannot go back forever: it has to start somewhere, from "first principles" that are true and not derived from anything more basic. The Stanford Encyclopedia of Philosophy's entry on Aristotle's logic explains that, for Aristotle, demonstration rests on starting points that are themselves not demonstrated.

In modern use, the idea is more practical. It is about separating what is genuinely fixed (physical facts, real constraints, clear goals) from what is merely habit, convention or someone's opinion.

First principles vs reasoning by analogy

Most of the time, we reason by analogy. "Other people do it this way, so I will too." That is efficient, and usually fine. You do not need to rethink how to boil an egg.

Analogy fails when the conventional answer is built on assumptions that no longer hold, or never did. First principles thinking is a tool for those moments. Ask:

• What is the actual goal here?
• Which parts of the usual approach are required by reality, and which are just tradition?
• If I were starting from nothing, what would I build?

The method in five steps

1. State the problem clearly. Write one sentence describing what you are trying to achieve, not how you plan to do it.
2. List your assumptions. Everything you currently believe about the problem, including "obvious" things.
3. Challenge each assumption. For each one, ask: how do I know this is true? Is it a fact, a rule someone made, or a habit?
4. Keep only the bedrock. Cross out anything that is convention or guesswork. What remains are your first principles: hard constraints, verified facts and your true goal.
5. Rebuild a solution. Ask what the simplest or best option would be, given only those principles. Then compare it with the usual approach.

Five examples of first principles thinking

Example 1: the cost of a product

A widely repeated example comes from Elon Musk, who described in an interview with Kevin Rose how he approached battery costs. As reported by CleanTechnica, people assumed battery packs would stay expensive because they always had been. Instead, he asked what batteries are made of and what those materials would cost on a commodity market. The materials turned out to cost far less than the finished packs, which suggested the price was driven by how batteries were made, not by any physical limit.

Whatever you think of the person, the reasoning pattern is useful: separate the cost of the ingredients from the cost of the process, and ask whether the process has to be that way.

Example 2: cooking without a recipe

A recipe is reasoning by analogy: follow these steps and you get this dish. A cook who understands first principles knows why the steps work. Salt draws out moisture and seasons. Heat browns food and changes its texture. Acid brightens flavour. Fat carries flavour.

Once you understand those principles, you can cook with whatever is in the fridge, fix a dish that tastes flat (it often needs salt or acid), and stop depending on exact ingredient lists.

Example 3: a personal budget

Conventional thinking says: look at last month, trim a bit here and there. First principles thinking starts elsewhere:

1. What do I actually need each month to be housed, fed, warm, healthy and able to work?
2. What do I value enough to pay for?
3. Everything else is optional by default.

Many people find that costs they treated as fixed, such as a car, a phone contract or a subscription bundle, were habits rather than requirements.

Example 4: a career decision

"I studied law, so I should be a lawyer" is reasoning from the past. From first principles, you might ask:

• What do I want my days to involve?
• What skills do I have that people pay for?
• What income do I genuinely need?
• What are the real costs and risks of switching?

The answers might still point to law. The point is that you have chosen it, rather than defaulted into it.

Example 5: a belief you inherited

First principles thinking also applies to ideas. Pick a belief you hold strongly and ask: where did it come from? What evidence supports it? What would change my mind? You may keep the belief, but now you hold it for your own reasons.

Common mistakes

• Using it for everything. Rethinking every decision from scratch is exhausting. Save it for problems that matter or feel stuck.
• Mistaking opinions for principles. "People will never pay for this" is a guess, not a fact.
• Ignoring expertise. Conventions often exist for good reasons. Understand why something is done before you discard it.
• Stopping at the analysis. Breaking a problem down is only useful if you rebuild and test a better solution.

Key takeaways

• First principles thinking means reasoning from basic, verified truths instead of copying convention.
• The idea dates back to Aristotle, who argued that knowledge must rest on starting points not derived from anything more basic.
• The method is: define the goal, list assumptions, challenge them, keep only the facts, and rebuild.
• It works on products, cooking, budgets, careers and beliefs alike.
• Use it selectively, for problems where conventional answers are not working.

If you want to apply this kind of reasoning to inherited beliefs, Thinking for Yourself: A Clear Case for Living Without Religion includes sections on how beliefs get installed, evidence and the burden of proof, and doubt as a method. You can also explore the wider Asendify library of practical playbooks.

FAQ

Who invented first principles thinking?

The concept is usually traced to Aristotle, who described first principles as the starting points from which knowledge is demonstrated. Later thinkers, including Descartes, developed related ideas.

Is first principles thinking the same as critical thinking?

They overlap. Critical thinking is the broader skill of evaluating claims and evidence. First principles thinking is one specific method within it: breaking a problem down to fundamentals and rebuilding.

How can I practise first principles thinking?

Take one everyday decision a week, write down every assumption behind it, and challenge each one. Over time, you will notice assumptions faster.

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