> You can tell someone their idea is substandard without inferring their stupid, which is generally taken to be an insult. Tact in communication does matter. I don't think anyone needs to say "that is just stupid" to get a point across.

What's wrong with calling an idea stupid? A smart person can have stupid ideas. (Or, more trivially, the person delivering a stupid idea might just be a messenger, rather than the person who originally thought of the idea.)

Though, to be clear, saying that an idea is stupid does carry the implication that someone who often thinks of such ideas is, themselves, likely to be stupid. An idea is not itself a mind that can have (a lack of) intelligence; so "that's stupid" does stand for a longer thought — something like "that is the sort of idea that only a stupid person would think of."

But saying that an idea is stupid does not carry the implication that someone is stupid just for providing that one idea. Any more than calling something you do "rude" when you fail to observe some kind of common etiquette of the society you grew up in, implies that you are yourself a "rude person". One is a one-time judgement of an action; the other is a judgement of a persistent trait. The action-judgements can add up as inductive evidence of the persistent trait; but a single action-judgement does not a trait-judgement make.

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A philosophical tangent:

But what both of those things do — calling an idea stupid, or an action rude — is to attach a certain amount of social approbation or shame to the action/idea, beyond just the amount you'd feel when you hear all the objective reasons the action/idea is bad. Where the intended response to that "communication of shame" is for the shame to be internalized, and to backpropagate and downweight whatever thinking process produced the action/idea within the person. It's intended as a lever for social operant conditioning.

Now, that being said, some people externalize blame — i.e. they experience "shaming messaging" not by feeling shame, but by feeling enraged that someone would attempt to shame them. The social-operant-conditioning lever of shame does not work on these people. Insofar as such people exist in a group, this destabilizes the usefulness of shame as a tool in such a group.

(A personal hypothesis I have is that internalization of blame is something that largely correlates with a belief in an objective morality — and especially, an objective morality that can potentially be better-known/understood by others than oneself. And therefore, as Western society has become decreasingly religious, shame as a social tool has "burned out" in how reliably it can be employed in Western society in arbitrary social contexts. Yet Western society has not adapted fully to this shift yet; which is why so many institutions that expect shame to "work" as a tool — e.g. the democratic system, re: motivating people to vote; or e.g. the school system, re: bullying — are crashing and burning.)