[ajuc]

What is missing is understanding that in math the sentence "if X = 3 then 2 * X = 6" is true even when X = 5.

For the proof to work the implication must be true, not the assumption.

[na85]

But you're not trying to prove X = 3 and starting out by assuming X = 3.

There wasn't even an attempt to disprove the assumption in TFA.

[ajuc]

> But you're not trying to prove X = 3 and starting out by assuming X = 3.

No, you're trying to prove

    for all N>=1: A(N) is true

and start by proving that

    IF A(N) is true THEN A(N+1) is true

This implication can be true even if A(N) is false, and your final proof will only use the implication, not the assumption used when proving that implication.

Assumptions in math proofs can have scopes. For example in many proofs you split the domain into subsets and prove that something is true assuming X>=0 and X<0 separately. Naively you would say that we cannot assume X>=0 and then assume X<0 in the same proof because that's a contradicion.

But these assumptions were used in different scopes so there's no contradiction.

[na85]

We're talking past each other. I give up.