 There are many ways the world could have been. You could have gotten up later today. Your parents could have failed to meet, so that you were never born. Life could never have developed on earth. The history of the universe could even have been completely different from the beginning. And many philosophers believe that the laws of nature could have been different as well (although that has been denied, as discussed in  section 2).  Maximally specific ways the world could have been are commonly called ‘possible worlds.’ The apparatus of possible worlds allows us to introduce a set of modal notions: a proposition is necessary just in case it is true in all possible worlds, a proposition is possible just in case it is true in some possible worlds, and it is contingent just in case it is true in some but not all possible worlds. A sentence is necessary (possible, contingent) just in case it expresses a necessary (possible, contingent) proposition.