How did G.E. Moore defend commonsense realism about the external world?

G.E. Moore defended commonsense realism by asserting that certain everyday beliefs are known with absolute certainty, making them more reliable than the complex, skeptical arguments used to challenge them.12

In his seminal essay, A Defence of Common Sense (1925), Moore argued that he and other human beings possess certain knowledge of propositions such as "The Earth has existed for many years" and "Many human beings have existed in the past". Because these propositions entail the existence of material objects, Moore maintained that any philosophical system—such as idealism or skepticism—that contradicts these basic beliefs can be rejected out of hand. He challenged skeptics to demonstrate that their premises were more certain than these fundamental, everyday convictions.21

Additionally, in his 1939 lecture "Proof of an External World," Moore famously displayed his hands to an audience, stating, "Here is one hand, and here is another." He argued that this simple demonstration satisfied the requirements for a proof of an external world, using it to raise pointed questions about why such a direct, commonsense observation would not be considered a valid refutation of skepticism.3