 This entry is about impossibilities. In order to read it fruitfully, you might also want to consult the entry on  possible worlds.  Many accounts of impossible worlds are best understood against the background of possible worlds theories — although symmetries and similarities can sometimes break down in important respects. (We will see one in  section 4  below.) You might also want to look at the entry on  modal logic  to grasp some basic notions of worlds semantics.