In 1900, German mathematician David Hilbert (1862–1943) proposed 23 unsolved mathematical problems for the new century, most of which only proved to bring up other problems. By the 1920s, Hilbert gathered many mathematicians—called the formalists—to prove that mathematics was consistent. But all did not go well as mathematical complications set in. By 1931, Kurt Gödel’s incompleteness theorem dashed any more efforts by the formalists by proving that mathematics is either inconsistent or incomplete. (For more about Hilbert, see “Foundations of Mathematics.”)