Final Exam: First-Principles Thinking

This is the final exam for First-Principles Thinking. It pulls together everything: reasoning by analogy versus first principles, the “why” ladder you use to drill to bedrock, telling assumptions apart from facts, the famous battery-pack rebuild, and — just as important — knowing when not to reach for first principles at all. Take your time and reason each question through; a few look easy until you spot the trap.

How this exam works

Read carefully — this exam is final. Each question appears one at a time. Once you submit an answer it is locked for good: there’s no going back, no retry, and no restart. Your score is hidden until the end, where you’ll see a pass/fail verdict. The pass mark is 70%. A few questions ask you to select all correct answers.

Question 1 of 22

What does it mean to "reason by analogy"?

You keep asking "why is that true?" until you reach bedrockYou decide what to do by copying what already worked for a similar situationYou rebuild your answer from facts you have personally verified to be trueYou refuse to use any prior knowledge and start from a blank page

Select an answer to continue.

Course Recap

Big picture

First-principles thinking, in one picture

First-principles thinking
- Analogy vs. first principles
  - Copy a known answer vs. rebuild from true facts
- The why-ladder
  - Ask "why is that true?" down to bedrock
- Assumptions vs. facts
  - Use the "how do I know this?" test
- The battery pack (decompose & rebuild)
  - $600/kWh assumption, ~$80/kWh material floor
- When NOT to use it
  - Slow & costly — save it for high stakes

Key takeaways

Reasoning by analogy copies a known answer — fast, cheap, and right most of the time, but it quietly inherits the example’s assumptions. First-principles thinking decomposes a problem to the truths you can verify and rebuilds from there; it doesn’t mean forgetting what you know, only refusing to let assumptions pose as facts. Use the why-ladder to drill to bedrock, the “how do I know this?” test to separate facts from inherited beliefs, and remember the battery pack: a rebuilt floor is a lower bound, not a price tag. Finally, know when not to bother — first principles is slow, so save it for high-stakes problems, failing standard answers, and assumptions that deserve questioning, and check Chesterton’s fence before you tear anything down.