All of John's friends say they know someone who has smoked 40 cigarettes a day for the past 40 years and yet who is r...

I've read most of the responses to these posts, and they all say we get to the correct answer by assuming "quite certain" means 100% true. If John's position is presented as a fact, how/why does that mean the position must be true? It can be 100% true that a person believes "x", but that would not force "x" itself to be 100% true. The key takeaway notes for this questions say when someone claims or says they believe something, that means there is doubt.