🪴 PG Notes

First Principles Thinking

5 min read

First Principles Thinking

Reasoning from fundamental truths rather than by analogy or convention. Breaking a problem down to its most basic elements and rebuilding from there.

How It Works

Instead of accepting received wisdom (“everyone knows X”), ask: What do I know to be true? Strip away assumptions, conventions, and analogies until you reach bedrock facts — then build back up.

Contrasted in High Agency in 30 Minutes with “last principles thinking” — starting from assumptions, treating them as unquestionable facts, and looking for confirming evidence. Last principles thinking is the root of the Attachment Trap.

Examples

  • wilbur-wright: “Birds fly → flight doesn’t defy physics → humans can fly → I can fly.” Rebuilt aerodynamics from scratch through wind tunnel experiments rather than accepting the consensus that flight was impossible.
  • Claude Shannon & Ed Thorp: “Roulette involves a physical ball on a physical wheel → physics governs the outcome → prediction is possible.” Built the first wearable computer.
  • david-deutsch’s asteroid thought experiment: With penguins, you can calculate fixed odds. With humans, you can’t — because human agency can change the outcome through first-principles problem solving.

naval-ravikant frames first-principles thinking as the mark of a “clear thinker”: real knowledge is built from the ground up, like understanding arithmetic before trigonometry. “If someone is using a lot of fancy words and a lot of big concepts, they probably don’t know what they’re talking about.” The test: can you explain it to a child? He cites richard-feynman’s “Six Easy Pieces” — building from the number line to precalculus through an unbroken chain of logic.

Munger: The Latticework

charlie-munger’s mental-models approach is first-principles thinking applied to decision-making across all domains. “If the facts don’t hang together on a latticework of theory, you don’t have them in a usable form.” His self-taught mastery of psychology, economics, and biology — assembling models from bedrock principles rather than accepting received disciplinary wisdom — is first-principles thinking as a lifelong practice.

Descartes: The Philosophical Founder

rene-descartes is the historical originator of the first-principles method in philosophy. In the Meditations (1641), he literally demolished all his beliefs and rebuilt from a single indubitable foundation — the cogito. His method of radical doubt is first-principles thinking taken to its logical extreme: reject everything that can be doubted, then build back up from what survives.

Mack’s high-agency framing and Naval’s “clear thinker” are both downstream of this 17th-century tradition, though they apply it pragmatically (in business, life) rather than epistemologically (in formal philosophy).

Sleep Science as First-Principles Example

The Borbély two-component model is a textbook case of first-principles thinking applied to biology. Rather than accepting folk wisdom about sleep (“early to bed, early to rise”), Borbély decomposed sleepiness into two independent physical processes: a circadian clock and a homeostatic hourglass. This decomposition explains why the same person can feel alert after 20 hours awake (circadian peak fighting homeostatic pressure) and sleepy after only 6 hours (circadian trough amplifying moderate pressure). Wozniak’s sleep article uses this decomposition to derive practical rules for optimising sleep-dependent learning — a direct parallel to how Shannon decomposed information into bits.

First Principles in Software Engineering

The Laws of Software Engineering collection implicitly frames first-principles thinking as one of its core decision tools. Several of the “laws” in that collection are what first-principles thinking produces when applied to engineering:

  • CAP Theorem — the rare case where first-principles reasoning was formalised into a proof. Brewer and Gilbert/Lynch reasoned from the physics of networked systems to derive that consistency, availability, and partition-tolerance cannot all coexist under partition. This is first-principles thinking applied to distributed systems.
  • Occam’s Razor — the Bayesian prior that supports first-principles conclusions: among hypotheses that fit the evidence, prefer the one with the fewest assumptions.
  • KISS and DRY — design principles that fall out of first-principles reasoning about maintenance cost: unnecessary complexity has compounding cost; redundant knowledge representations drift.
  • Hyrum’s Law — a first-principles observation about empirical coupling: every observable behaviour of a system is part of its contract, because users depend on what actually happens, not what you documented.

Conversely, most of the “laws” in the collection are what you get when first-principles thinking fails to be applied: goodharts-law is first-principles about incentives (measure what you want, not what’s easy); hofstadters-law is first-principles about estimation (use base rates, not the inside view); brooks-law is first-principles about team dynamics (communication scales as n², not linearly).

Connections

  • high-agency — First principles thinking is the foundation of the “clear thinking” component
  • inversion — A complementary technique: reason from what would make things worse
  • mental-models — Munger’s latticework is first-principles thinking systematized
  • judgment — Naval: clear thinking is the foundation of good judgment
  • behavioral-psychology — Cognitive biases are the enemy of first-principles reasoning
  • anchoring-bias — First principles thinking is the antidote to anchoring
  • framing-effects — Reframing from first principles reveals when you’re being manipulated by presentation
  • fallibilism — Deutsch: all knowledge is conjectural and improvable; first principles must themselves be revisable
  • cartesian-doubt — Descartes’ method is the philosophical ancestor of first-principles thinking
  • rene-descartes — The 17th-century originator of first-principles method in philosophy
  • two-component-sleep-model — BorbĂ©ly’s decomposition of sleepiness into circadian + homeostatic: first-principles biology
  • cap-theorem — The canonical first-principles theorem in distributed systems
  • occams-razor — First-principles’ epistemological companion: prefer the simplest explanation

Sources

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