Alonzo Church, (born June 14, 1903, Washington, D.C., U.S.—died Aug. 11, 1995, Hudson, Ohio), U.S. mathematician. He earned a Ph.D. from Princeton University. His contributions to number theory and the theories of algorithms and computability laid the foundations of computer science. The rule known as Church’s theorem or Church’s thesis (proposed independently by Alan M. Turing) states that only recursive functions can be calculated mechanically and implies that arithmetic procedures cannot be used to decide the consistency of statements formulated in accordance with the laws of arithmetic. He wrote the standard textbook Introduction to Mathematical Logic (1956) and helped found the Journal of Symbolic Logic, which he edited until 1979.