自然主义谬误(naturalistic fallacy)
视「善」（或任何相等的词语）彷佛它是自然属性的名称的谬误。1903年，摩尔在他的《伦理学原理》（Principia Ethica）一书中提出他的「问题待解的论证」（open-question argument），对抗他所谓的自然主义谬误，其目的在於证明，「善」是个简单的、无可分析的性质名称，它无法以世界上某些自然的性质加以界定，无论它是弥尔所言的「使人感到快乐」，或斯宾塞所言的「高度发展」。因为摩尔的论证应用在任何以其他事物来界定善的尝试，包括超自然者，如「上帝所愿」，因此「自然主义谬误」一词并不贴切。问题待解的论证使得任何对善提出的定义都陷入疑难。（举例来说，「善的意思是使人感到快乐」就变成「是否每一件使人感到快乐的事就是善？」）摩尔的要点是，如果问题是有意义的，则提议的定义就是不正确的，因为如果定义是正确的，则问题将是没有意义。
English version：
naturalistic fallacy
Fallacy of treating the term "good" (or any equivalent term) as if it were the name of a natural property. In 1903 G. E. Moore presented in Principia Ethica his "open-question argument" against what he called the naturalistic fallacy, with the aim of proving that "good" is the name of a simple, unanalyzable quality, incapable of being defined in terms of some natural quality of the world, whether it be "pleasurable" (John Stuart Mill) or "highly evolved" (Herbert Spencer). Since Moore's argument applied to any attempt to define good in terms of something else, including something supernatural such as "what God wills," the term "naturalistic fallacy" is not apt. The open-question argument turns any proposed definition of good into a question (e.g., "Good means pleasurable" becomes "Is everything pleasurable good?")-Moore's point being that if the question is meaningful, the proposed definition cannot be correct, since if it were, the question of would be meaningless.