 The two key concepts in possible worlds semantics are possible worlds and accessibility spheres (or relations). Intuitively, a possible world w is simply a way the world could be or could have been. Formally, they are treated as primitive points in the set of all possible worlds W. But their crucial role comes in assigning truth-conditions to sentences: a sentence \(\phi\) can only said to be true given a possible world w, but since w is genuinely possible, it cannot be the case that both \(\phi\) and \(\neg\phi\) are true at w. Accessibility spheres provide additional structure for reasoning about what’s possible: for each world w, \(R(w)\) is the set of worlds accessible from  w.[18]  This captures the intuitive idea that given a possible world w, a certain range of other worlds \(R(w)\) are possible, in a variety of senses. \(R_1(w)\) might specify what’s nomologically possible in w by including only worlds where w’s natural laws hold, while \(R_2(w)\) specifies what’s metaphysically possible in w.