Lesson 13 of 48
Deduction
Every step justified by a step already earned.
Proof is the discipline of never using something you have not paid for. It is also the exact habit that catches a plausible-sounding argument — from a person or from a model — that happens to be wrong.
Do this
Prove an early Euclid proposition with every line justified by a postulate, a definition, or a proposition already proved. Hand it to someone whose job is to find the unjustified line.
The question that unlocks the next lesson
In a deductive proof, what makes a step legitimate?
- AIt is obviously true when you look at the diagram
- BIt follows from a definition, postulate, or result already established
- CIt matches the answer in the back of the book
- DIt works for every example you tried