 Later developments have been primarily in the ‘Hilbertian’ wing of the formalist movement. P. J. Cohen’s work on the generalised continuum hypothesis showed, in tandem with Gödel’s relative consistency proof, that it and countless related set-theoretic propositions concerning the relations of the cardinality of a set to that of its powerset are undecidable by current axioms. With no obvious, non-ad hoc, ways to extend the axioms to decide these questions, this led some mathematicians, such as Cohen himself (Cohen, 1971) and Abraham Robinson, (Robinson, 1965; 1969) to despair of a realistic interpretation of higher set theory. They therefore treat branches of mathematics in which no plausible axiom set will decide the key questions as ‘ideal’ parts of mathematics, lacking the content to be found in other areas.