There is more than one solution of the dilemma just mentioned. One way is very simple indeed. Possible world models can be used to represent various kinds of modalities. They provide mathematical machinery, but they do not say what the machinery is for. That is up to the user. So, we might want to have such a model to represent necessary truth, or we might want to have such a model to represent epistemic issues. The argument that proper names are rigid designators applies to models representing necessary truth. It does not follow that this is the case for epistemic models too. Here is a quote from (Kripke 1980) that sheds some light on the issue.