It ' s stupid to think that anyone who disagrees with you is stupid. That runs contrary to the philosophical basis for knowledge, which is Fallibilism (and could be the base for terrorism). Anyone who honestly cares about truth must maintain a possibility of being wrong in all things, and much of the search for truth is assigning that probability based on current knowledge. Bayesian Epistemology But when you set something at 100%, you lose any openness to the possibility of being wrong whatsoever. Which means if you ARE wrong, you will never know? Which is tragic, to anyone who cares about truth? Now we can be nearly certain about all kinds of things that imply people who disagree are plainly wrong. But being plainly wrong doesn't make a person stupid. It just means that person is operating off of faulty axioms. Logic 101 is that a perfectly logical Syllogism can still be wrong, if your premises/axioms are wrong. Our minds work the same way. Steve McKerracher's answer to Rationality: How is it that the majority of the adult population are unable to represent their thoughts as clearly and rationally as many kindergarten students can? (Admittedly they have to rely on poor information, which is a very different problem) (My answer... they aren't.) This is precisely why reasonable discourse with people who disagree with you, is an absolutely vital element of an honest search for truth. It has the possibility of exposing faulty axioms for one or sometimes both parties. Even if have convincing evidence that their position is incredibly improbable, often you can still learn something. Or find a better way to be persuasive in your arguments. Or come to understand the other side better. Even Young Earth Creationists, who we know, as well as we can know ANYTHING, are wrong, have asked useful questions of Evolution, and Abiogenesis. For instance, they asked how life could possibly arise out of a chemical soup when half of random molecules are spinning in opposite directions, and all life is composed of only molecules spinning in the same direction. (Found to be not entirely true actually.) But the question prompted the research that seems to have answered that dilemma: Most importantly, thinking people are stupid will alienate them, and frustrate you. I know it's hard to keep from slipping into that mindset, when you know people are passionately defending a stupid belief, but that doesn't make the person stupid, and it is in no way beneficial to think that way.