Alan Turing is being honored with a Google doodle this weekend on the occasion of the 100th anniversary of the birth of the pioneering British computer scientist and father of artificial intelligence.

Turing (June 23, 1912-June 7, 1954) worked as a code breaker during World War II, heading the team tasked with cracking German naval codes at Britain’s Government Code and Cypher School (GCCS). Prior to the war while a student at Cambridge, he developed his famous “Turing machine,” a variation of which is depicted in Google’s animated Turing doodle below.

A Turing machine is not an actual computer but rather a hypothetical one that still serves as a fundamental tool for understanding how algorithms, computer programming, and computing itself works. Turing described his conceptual computer, which he referred to as a Logical Computing Machine, in his 1948 essay “Intelligent Machinery” as having:

“…an unlimited memory capacity obtained in the form of an infinite tape marked out into squares, on each of which a symbol could be printed. At any moment there is one symbol in the machine; it is called the scanned symbol. The machine can alter the scanned symbol and its behavior is in part determined by that symbol, but the symbols on the tape elsewhere do not affect the behavior of the machine. However, the tape can be moved back and forth through the machine, this being one of the elementary operations of the machine. Any symbol on the tape may therefore eventually have an innings.”

Turing did help design and build functional computational machines in the 1940s and 50s, including groundbreaking experimental computers like the Automatic Computing Engine (ACE) while working at Britain’s National Physical Laboratory and the Manchester machines at the Royal Society Computing Machine Laboratory run by the famous mathematician and code breaker Max Newman at Manchester University.

But he is most famous today for the “Turing test.” This proposed method for determining if a machine can “think” is considered the basis of the science of artificial intelligence.

There is actually some controversy over how Turing proposed to test machines for intelligence in his 1950 essay “Computing Machinery and Intelligence.” In the essay, Turing describes question-and-answer games that involve a “blind” player interrogating two other players, one a human and one a computer, to try to determine the gender of each. In different versions of the game, the players attempt to either trick or assist the interrogator in making his or her final determination of gender.

Turing proposes that if the interrogator is as often right (or wrong) about the computer’s gender as the human’s, then the computer can be described as “intelligent.” However, Turing tests have evolved into a form used today known as the standard Turing test where the interrogator is attempting to simply determine which of the two players being questioned is a human and which is a computera formulation of the game that some argue Turing intended while others believe he did not.

The annual Loebner Prize competition, initiated in 1990 by the American inventor Hugh Loebner, uses a standard Turing test on computer programs entered in the contest to determine which is the most human-like.

Turing was gay during a time when homosexuality was persecuted under the law in the U.K. He was convicted of illegal homosexual acts in 1952 and forced to undergo chemical castration treatment to avoid a prison term.

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