It was in 1963 that we were hit by a real bomb, however, when Paul J. Cohen discovered his method of ‘forcing’… Set theory could never be the same after Cohen, and there is simply no comparison whatsoever in the sophistication of our knowledge about models for set theory today as contrasted to the pre-Cohen area. One of the most striking consequences of his work is the realization of the extreme relativity of the notion of cardinal number. Dana Scott in (Bell 2005, p.xiv)

From a broader perspective Cohen’s results fit nicely into the landscape of relativising set theory and the rise of ‘variable sets’in the work of Grothendieck, Lawvere and Tierney on topos theory in the 60s and afterwards.