You just make priors up, just like the 0.05 limit is made up. Making up a prior is basically asking the question: How likely does it seem that X has Y effect? W

You just make priors up, just like the 0.05 limit is made up. Making up a prior is basically asking the question: How likely does it seem that X has Y effect? Without having any data specific to that question. Our 0.05 p-value limit is effectively just answering that question with a fixed 5%, no matter how ridiculous the proposition is.