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How did G.E. Moore defend common sense against skepticism and idealism?

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Four colored straws in a glass of water appear bent at the water's surface, visually demonstrating the refraction of light. — Epistemology The refraction (bending) of light as it passes from air into water causes an optical illusion: straws in the glass of water appear broken or bent at the water's surface.

G.E. Moore defended common sense against skepticism and idealism by asserting that certain basic propositions about the world are known with absolute certainty. In his influential 1925 paper, A Defence of Common Sense, Moore argued that human beings possess certain knowledge of facts, such as "the Earth has existed for many years" and "many human beings have existed in the past".12

Moore’s defense functioned as follows:
* Against Skepticism: He maintained that because we know these basic propositions to be true, skeptical claims—which suggest that nobody knows anything to be true—can be dismissed.12
* Against Idealism: Because these certain propositions entail the existence of material objects, Moore argued that idealism, which posits that the world is entirely mental, must be rejected.1

In his 1939 lecture, "Proof of an External World," Moore famously demonstrated this by holding up his hands and stating, "Here is one hand, and here is another." He argued that this simple act satisfied the requirements for a proof of an external world, insisting that any philosophical system contradicting such common-sense certainties could be rejected without further analysis.31