How did G.E. Moore challenge philosophical skepticism?

G.E. Moore challenged philosophical skepticism by championing a "common-sense" view of the world, which he argued was known with certainty by all human beings. In his influential paper "A Defense of Common Sense" (1925), Moore asserted that propositions such as "the Earth has existed for many years" and "many human beings have existed in the past" are known to be true.123

Because these basic propositions are certain, Moore argued that any philosophical system—including skepticism—that contradicts them can be rejected out of hand. He maintained that skeptical arguments, regardless of how compelling they may seem, cannot undermine this foundational knowledge.123

Moore further illustrated this stance in his 1939 lecture, "Proof of an External World," where he famously displayed his hands to an audience, stating, "Here is one hand, and here is another." By presenting this as a valid proof of an external world, Moore intended to raise a pointed question about why such a trivial, common-sense observation should not satisfy the requirements for proof, thereby challenging the skeptical demand for more rigorous justification.4