 Logicist doctrines were espoused in two main forms—Fregean and Russellian—until around 1930, at which point logicism went into decline, largely because of the discovery of Gödelian incompleteness, and the ascendancy of Zermelo-Fraenkel set theory, which displaced the Russellian theory of types as the most promising foundational theory for mathematics. The doctrine of neo-logicism subsequently revived some of the core ideas of logicism, with its first hints appearing in the mid-1960s, and its more substantive contributions beginning in the 1980s. 