IMO, perfect reasoning is rare. I think almost everyone has some minor flaws in reasoning but can still reach conclusions that allow them to function in society. Multiple minor flaws can magnify each other and major flaws can derail reasoning badly enough to make people reach conclusions that keep them from functioning in a way that others can deal with or conclusions that place themselves or others in danger. I think madness is fatally flawed reasoning, not the absence of reasoning. I also think most people don't abandon reasoning, reasoning fails them due to flaws they are unaware of (of, course, ignorance of the flaw is no excuse ). I think that was just clever dramatic dialog.