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How did G.E. Moore attempt to prove the existence of an external world?

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Marble bust of Aristotle with a beard and draped robe, showing detailed facial features and wavy hair, set against a dark background. — metaphysics Detail of a Roman copy (2nd century bce) of a Greek alabaster portrait bust of Aristotle (c. 325 bce); in the collection of the Museo Nazionale Romano, Rome.

In his 1939 lecture, "Proof of an External World," G.E. Moore presented a famously blunt argument to demonstrate the reality of an external world. He held up one hand and stated, "Here is one hand," then held up his other hand and added, "and here is another." Moore argued that this simple demonstration satisfied all reasonable requirements for a formal proof, as he and his audience knew with certainty that a pair of hands was present before them.12

Moore’s approach was rooted in his "common-sense philosophy," which maintained that certain propositions about the world are known to be true with absolute certainty. By asserting that such knowledge exists, he sought to dismiss skepticism and idealism, arguing that any philosophical system contradicting these basic convictions could be rejected without further analysis.34

While some critics viewed his proof as absurd, it is often interpreted not as a traditional attempt to defeat skeptics, but as a way to challenge the philosophical demand for such proofs and to highlight the legitimacy of common-sense knowledge.12