[thawaway1837]

I think this is true of any “answering engine” in an ideal world. In an ideal world, it would act as a function and return a single output for any input, because any particular question should ideally only have a single correct answer.

[CM30]

How would it decide which source to get this answer from?

Because let's face it, one of the biggest challenges with that idea isn't that there are tons of different valid answers to a question (though that could be the case for many political queries or ones related to art/media quality), but that hundreds or thousands of pages and sites contain the requested info, and search engines have to rank them in some sort of order.