I try to always include an extra option which is just "everything I haven't thought of yet", and the prior for that option reflects how confident you feel about your mastery of the domain. Even a medical expert must always admit some small likelihood that the true cause is neither a tumor nor a cold, because medical experts know that medical science evolves over time.
That is good. But how would you assign the probability of 'everything you haven't thought of yet' in a problem? You would have to base it again on data; which takes us back to the same question. Data can of course be sufficient for fields that are advanced to a point of overwhelming clarity. But when we get into sufficiently complex subjects the rational approach would be to say that my 'everything I haven't thought of yet' prior is so large that the only rational answer is "I don't know". Would you agree?
This thread is for asking any questions that might seem obvious, tangential, silly or what-have-you. Don't be shy, everyone has holes in their knowledge, though the fewer and the smaller we can make them, the better.
Please be respectful of other people's admitting ignorance and don't mock them for it, as they're doing a noble thing.
To any future monthly posters of SQ threads, please remember to add the "stupid_questions" tag.