Student or Learner
Mel is asking the boy if he saw his dead sister at some house, and he answers like this, and Mel responds like "What would you say that?"
Is this "would" a presumption or an indirect politeness? I think it's a presumption for she is much older than the boy.
boy - I think she's still there, too... Emily.
Mel- Why would you say that?
boy- I saw her. Emily!
It's not presumption if it's a question.
It has more of the idea of, "That is a strange thing to say", It has the strangeness of, "If you were going to say something, why would it be that? (hypothetical - but he did say it.)
To me, it's like What makes you say that?
I encountered numerous "would" phrases such as "Why would they do it?" for "I wonder why young people do such a strange thing." or "Why would you encourage him?" for "I wonder why you encourage him to do it when you are expected not to do that".
So "would" seems an presumptional question in this case meaning "I wonder why you would do it when you are expected not to do that, it's quite strange", so it seems a little bit different from the basic presumption phrases.
I really don't see how the 'presumptional' label fits here. One of the problems here, keannu, is that you appear to be trying to slot all uses of would' into a few pigeonholes with neat labels. You can't. Modal verbs are notoriously 'messy'.
will/would, frequently, but not always, convey ideas of certainty, volition, determination, habitual characteristic, etc.
would frequently, but not always appears to be used as a distancing form of will.
Unfortunately for grammarians, will and would are sometimes used in other ways that can be described, but not easily explained.
"If you were going to say something, why would it be that? (hypothetical - but he did say it.)
If, when I saw you last week, you were going to say (later that day) that you didn't like the idea ...
If, when I saw you last week, you were going to say (tomorrow) that you didn't like the idea, ...
These are pretty contrived examples, (particularly as, in my original example, the counterfactual actually happened!) but they do illustrate some of the difficulties sometimes involved in the questions, "Is it possible to say ...?" and "What does ... mean?".
I have just re-read my last post, and I don't like it. I am leaving it, because it is a good illustration of the problems involved here. It is very difficult to say exactly why we use 'would' in certain examples. This may cause problems for learners who want to have an exact explanation of everything they encounter, but native speakers know what is meant. The simplest answer here is Tdol's, in post #4..
I must also add, that the example sentence is far more likely, in my opinion, to be expressed as Tdol did, or as 'Why do/did you say that?"
I guess, this "would" has a hypothetical meaning, like "I think you wouldn't say it, but due to what, would you say it?" By saying "what you said is unlikely to happen", you are emphasizing it's weird or strange.