 As noted above, it was suggested already in the 1920s that the CH might be independent of ZF or ZFC. After first conjecturing that the Axiom of Constructibility might be “absolutely consistent,” meaning not falsifiable by any further extension of models of ZF + V =  L,[17]  in his 1947 “What is Cantor’s Continuum Hypothesis?” Gödel conjectured that the CH would be shown to be independent. The main consequence of Gödel’s result, then, as far as the problem of proving the independence of the CH is concerned, was that it pointed mathematicians in the direction of adding non-constructible sets to a model of set theory in order to establish the consistency of the negation of the CH. In 1961 Dana Scott proved that the failure of the Axiom of Constructibility follows from the existence of a measurable cardinal, contrary to a conjecture Gödel had made in 1940. (See Scott 1961. A cardinal κ is said to be measurable if there is a non-principal κ-complete ultrafilter in the power-set Boolean algebra of κ.) In 1963, as noted, Paul Cohen proved the consistency of the negation of the CH by adding non-constructible sets to an inner model.