I'm having trouble understanding what possibility means in modal logic. The Wikipedia page gives "It is possible that it will rain today." as an example of a statement which is possible. But in this case "possible" just means that we don't know whether it's going to rain or not.
Can't "$P$ is possible" be defined to mean "We don't know $P$ and we don't know $\lnot P$", making modal and epistemic logic equivalent? Or is there something I'm missing?