What makes appeal to authority fallacious is that one's supposed expertise has no bearing on the truth of the things one says. An appeal to authority is when s

What makes appeal to authority fallacious is that one's supposed expertise has no bearing on the truth of the things one says. An appeal to authority is when someone argues something like, "I'm an expert, I think X and so should you" basing their opinion on X on their supposed expertise and not actual logic. When someone gives reasons, their argument rests on those reasons, not on their supposed authority as an expert on the topic.