He is noted for inventing a technique called forcing which he used to show that neither the continuum hypothesis nor the axiom of choice can be proved from the standard Zermelo-Fraenkel axioms of set theory. In conjunction with the earlier work of Gödel, this showed that both these statements are independent of the Zermelo-Fraenkel axioms: they can be neither proved nor disproved from these axioms. For his efforts he won the Fields Medal.
This result is possibly the most famous non-trivial example illustrating Gödel's incompleteness theorem.
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