Sivers has it right, and I'm disappointed by the reaction here. People are smart. This should be your baseline. From there, people may do or say stupid things. It may become pathological. But the person has not lost his capacity to be smart (barring some physiological damage). It may have atrophied, like an unused muscle, but the capacity is still there.
But even this is getting ahead of yourself--how would you know they are stupid? Acting stupid is at least multi-dimensional: the act+stupid. It's a limited claim. To be stupid, though, now that's a very broad claim, to reduce things to a position on a single axis. And that is pure hubris, an intellectually indefensible position, an argument from apathy: "well I don't care about all those things, to me the guy's just dumb". Ah well thanks for clearing that up!
And there's the inverse: you casually judge others as stupid, and you have shown yourself to have poor judgment. You are being stupid with judgment.
If I go along with the premise that people are merely "acting" stupid, that is all the more reason to casually dismiss them. I can have patience with people that have limited capabilities, but people who choose to act stupid?
Being semantically correct isn't particularly relevant. It's how you deal with consistent stupidity (and I'm not talking about the occasional brainfart or error of judgement).
Whether or not casually dismissing stupidity is the right thing to do depends completely on the context, not on whether you use the correct semantics to do so. I people are being viciously stupid (as in "gay people shouldn't have equal rights"), I will dismiss them as stupid, and I don't give a flying fuck if they are acting or being stupid.
And on the flip side there are also situations in which it is useful to figure out why people do or say stupid things. It's context that determines if my judgement is poor, not semantics.
I can have patience with people that have limited capabilities, but people who choose to act stupid?
You assume it's a choice, as if at the brink of decision, someone says, "I'll do the stupid thing." But in real life, you see people motivated by ambition, pride, fear and desire. And if that drives you do stupid things, to argue for stupid positions, does that make you stupid? Well, if limit ourselves to "stupid" always being contextual, if we understand "is stupid" as "acts stupid in a given context", then sure.
Likewise, if someone doesn't care about the things you care about, and consequently doesn't focus their attention on them to the same degree you might, does that make them stupid? Or just uninterested?
It's not a semantic issue. You are dismissing people outright, you are creating an alternate universe in your mind that through hubris drifts further and further from reality. Invariably, there's something else going on, something more interesting and more true to reality than just "he's stupid", and you miss that. I'm not arguing that you necessarily should engage with such people, but neither should you box them off with pat pronouncements on intellectual capacity.
It is axiomatic. Maybe I could prove it directly, but as an axiom, it is more productive than "some people are smart, some people are stupid".
For example, when you look at children, or look at history, or look at other cultures, you see patterns that make more sense when you start with this axiom. It's not perfect, but if you have a concept of "smart", it is better to assume all people are smart, to take it off the table entirely, than to assume some are smart and some are stupid. The latter will shortcut your reasoning: "Oh well they must just be stupid."
But even this is getting ahead of yourself--how would you know they are stupid? Acting stupid is at least multi-dimensional: the act+stupid. It's a limited claim. To be stupid, though, now that's a very broad claim, to reduce things to a position on a single axis. And that is pure hubris, an intellectually indefensible position, an argument from apathy: "well I don't care about all those things, to me the guy's just dumb". Ah well thanks for clearing that up!
And there's the inverse: you casually judge others as stupid, and you have shown yourself to have poor judgment. You are being stupid with judgment.