  Only clauses 6 and 7 rely crucially on this richer notion of meaning. \({{\medsquare}}\phi\) says that in all accessible worlds \(R(w)\), \(\phi\) is true. \({{\meddiamond}}\phi\) says that there are some accessible worlds where \(\phi\) is true. Logical concepts like consequence are also defined in terms of relations between sets of possible worlds. The intersection of the premises must be a subset of the conclusion (i.e., every world where the premises are true, the conclusion is true):