  The inner model L is the union of all such stages: L = ∪α∈On Lα. Gödel showed that L satisfies (arbitrarily large fragments of) ZFC along with CH.  It follows that ZFC cannot refute CH. Cohen complemented this result by inventing (in 1963) the method of forcing (or outer models). Given a complete Boolean algebra B he defined a model VB and showed that ¬CH holds in VB.[6] This had the consequence that ZFC could not prove CH.  Thus, these results together showed that CH is independent of ZFC.  Similar results hold for PM and a host of other questions in set theory.