How does G.E. Moore’s open question argument challenge attempts to define moral goodness in natural terms?

In his work Principia Ethica (1903), G.E. Moore utilized the "open-question argument" to challenge ethical naturalism, which attempts to define moral goodness in terms of natural properties like pleasure or utility.123

Moore argued that "good" is a simple, unanalyzable quality. His argument functions by testing any proposed naturalistic definition of "good." If one defines "good" as "pleasurable," Moore asks, "Is everything pleasurable good?" He contended that if this question remains meaningful—meaning a negative answer is not a logical contradiction—then the definition is incorrect. If "good" truly meant "pleasurable," the question would be as incoherent as asking, "Is this unmarried man a bachelor?"132

By demonstrating that such questions remain "open" even to those who understand the terms, Moore aimed to show that naturalistic definitions fail to capture the full meaning of "good". Consequently, he concluded that moral goodness cannot be reduced to natural facts and must instead be apprehended through intuition.124