tjboudreaux/thinking-first-principles

First Principles Reasoning

Overview

First principles thinking strips away assumptions and conventions to reveal fundamental truths, then reconstructs solutions from those basics. This approach, championed by Elon Musk and rooted in Aristotle's philosophy, enables breakthrough solutions by escaping the trap of reasoning by analogy.

Core Principle: Don't accept "that's how it's always done." Reduce to fundamentals, then rebuild.

When to Use

• Conventional approaches have failed or seem inadequate
• You're told something is "impossible" or "too expensive"
• Designing new systems/products from scratch
• Challenging industry assumptions or pricing
• Problem seems intractable using existing mental models
• You need innovation rather than incremental improvement

Decision flow:

Problem intractable? → yes → Are you reasoning from analogy? → yes → APPLY FIRST PRINCIPLES
                                                            ↘ no → Already at fundamentals
                  ↘ no → Standard problem-solving may suffice

When NOT to Use

• The constraint is genuinely physics/cost/regulation, not convention — verify, then accept it and move on.
• A well-trodden, working solution already exists; reinventing from scratch adds risk without lift. Use the convention.
• Routine implementation where the "why" is settled — don't re-derive a standard library, protocol, or known-good pattern.
• Time-critical incidents: act on the most likely cause first (occams-razor), reserve first-principles for the post-incident redesign.

Trigger Card

When a constraint is treated as fixed ("too expensive", "impossible", "always done this way"):

1. Ask: is this constraint physics or convention? If it's a law of nature, accept it. If it's "how we've always done it," it's negotiable.
2. Identify the irreducible truths — what must be true no matter what solution you pick?
3. Rebuild from those truths — derive a solution that satisfies the real constraints but ignores the artificial ones. If a simpler rebuild solves it, stop.

Skip if the "why" is settled (standard library, known-good pattern). In an incident, act on the most likely cause first; reserve this for post-incident redesign.

The Process

Step 1: Identify Current Assumptions

List everything you "know" or assume about the problem:

• What are the constraints everyone accepts?
• What costs/limitations are considered fixed?
• What's the conventional wisdom?
• Why do people say it can't be done?

Example: "Rocket launches cost $65M because that's what aerospace companies charge"
Assumptions: Must buy from existing suppliers, existing designs are optimal, 
             materials must be aerospace-grade, vertical integration is too hard

Step 2: Break Down to Fundamentals

Ask repeatedly: "What are we absolutely certain is true?"

Decomposition questions:

• What are the physical/mathematical constraints?
• What are the raw inputs required?
• What laws of physics/economics apply?
• What would this cost if built from raw materials?
• What is the minimum viable version?

Example: Rocket raw materials (aluminum, titanium, copper, carbon fiber)
         = ~2% of typical rocket price
Fundamental truth: Materials aren't the cost driver; 
                   manufacturing/overhead/margins are

Step 3: Challenge Each Assumption

For each assumption, ask:

• Is this actually true, or just convention?
• What evidence supports this?
• Under what conditions would this be false?
• Who benefits from this assumption persisting?

Use the "5 Whys" to drill deeper:

Why is it expensive? → Suppliers charge high margins
Why? → Limited competition
Why? → High barriers to entry
Why? → Assumption that you must use existing supply chain
Why? → Nobody questioned it

Step 4: Rebuild from Fundamentals

Starting ONLY from verified truths:

• What's the simplest solution that satisfies fundamental requirements?
• What new approaches become possible without false constraints?
• How would you solve this if starting from scratch today?
• What would a solution look like with 10x less resources?

Example: Build rockets in-house using commodity materials
         = SpaceX reduced launch costs by 10x

Step 5: Validate Against Reality

• Does your reconstructed solution violate any physics/laws?
• What practical constraints remain?
• What's the minimum viable test?
• How can you prove/disprove this quickly?

Mental Traps to Avoid

| Trap | Description | Antidote | |------|-------------|----------| | Reasoning by Analogy | "Others do it this way" | Ask "but is it optimal?" | | Appeal to Authority | "Experts say it's impossible" | "What specifically makes it impossible?" | | Sunk Cost | "We've always done it this way" | "What if we started fresh today?" | | Complexity Bias | Assuming complex = better | "What's the simplest version?" | | False Constraints | Accepting artificial limits | "Is this a law of physics or convention?" |

Application Examples

Software Architecture

Assumption: "We need a microservices architecture"
First Principles:
- What problem are we solving? (scalability, team independence, deployment)
- What's the minimum that achieves this?
- A modular monolith might suffice for current scale

Cost Reduction

Assumption: "This service costs $50k/month, that's just cloud pricing"
First Principles:
- What compute/storage do we actually need?
- What are we paying for that we don't use?
- Could we reduce by 80% with right-sizing?

Feature Development

Assumption: "Users need feature X (because competitor has it)"
First Principles:
- What job is the user trying to accomplish?
- What's the simplest way to enable that job?
- Maybe a different approach solves it better

Verification Checklist

• [ ] Listed all assumptions about the problem
• [ ] Identified which assumptions are physics/math vs convention
• [ ] Challenged at least 3 "obvious" constraints
• [ ] Rebuilt solution using only verified fundamentals
• [ ] Validated that solution doesn't violate actual constraints
• [ ] Identified minimum viable test to prove/disprove approach

Combining with Other Models

• Inversion: After first principles, ask "what would make this fail?"
• Second-Order Thinking: Consider downstream effects of your new approach
• Pre-Mortem: Imagine your first-principles solution failed—why?

Key Questions

• "What do we know to be absolutely true?"
• "Why does everyone assume this constraint exists?"
• "What would we do if we had to solve this with 10% of the budget?"
• "If we were starting from scratch today, would we build it this way?"
• "What would a complete outsider try?"