G. E. Moore: Principia Ethica
According to G. E. Moore, the fundamental task of moral philosophy is to provide a definition of goodness. In seeking a definition, Moore is not simply trying to discover what it is that most people mean when they say the word good, nor does he aim to stipulate his own novel definition. Rather, he seeks to give an account of the nature of goodness by analyzing the concept in terms of its component parts, much as one might define bachelor as an unmarried man. According to Moore, however, "good" cannot be defined in this sense. This is because the concept of goodness is simple-it has no parts. Moore claims that in this respect "good" is like "yellow": You cannot explain what the concept is to someone who does not already know it.
Moore argues that many previous philosophers have failed to recognize this point, and in doing so have committed what he calls the naturalistic fallacy, the mistake of believing that goodness is identical to some natural property, such as happiness or that which we desire. (Elsewhere, Moore uses a discipline definition of "natural" as a property that is appropriately studied by those working on the natural sciences.) Against any such identity claim, Moore deploys what has become known as the open question argument. For any proposed definition of good, Moore claims, it is always sensible to ask whether things of that sort are good. For instance, the question "Is pleasure good?" appears to be a perfectly sensible question to ask - it is an open question. In contrast, the question "Is pleasure pleasant?" is trivial - it is a closed question. According to Moore, this proves that goodness and pleasure cannot be the same thing. Moore holds that this test can be used to disqualify any proposed definition of goodness, and thus goodness cannot be identical with any natural property.
-According to Moore, the good is indefinable.

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The Answer of G. E. Moore: Principia Ethica
According to G. E. Moore, the fundamental task of moral philosophy is to provide a definition of goodness. In seeking a definition, Moore is not simply trying to discover what it is that most people mean when they say the word good, nor does he aim to stipulate his own novel definition. Rather, he seeks to give an account of the nature of goodness by analyzing the concept in terms of its component parts, much as one might define bachelor as an unmarried man. According to Moore, however, "good" cannot be defined in this sense. This is because the concept of goodness is simple-it has no parts. Moore claims that in this respect "good" is like "yellow": You cannot explain what the concept is to someone who does not already know it.
Moore argues that many previous philosophers have failed to recognize this point, and in doing so have committed what he calls the naturalistic fallacy, the mistake of believing that goodness is identical to some natural property, such as happiness or that which we desire. (Elsewhere, Moore uses a discipline definition of "natural" as a property that is appropriately studied by those working on the natural sciences.) Against any such identity claim, Moore deploys what has become known as the open question argument. For any proposed definition of good, Moore claims, it is always sensible to ask whether things of that sort are good. For instance, the question "Is pleasure good?" appears to be a perfectly sensible question to ask - it is an open question. In contrast, the question "Is pleasure pleasant?" is trivial - it is a closed question. According to Moore, this proves that goodness and pleasure cannot be the same thing. Moore holds that this test can be used to disqualify any proposed definition of goodness, and thus goodness cannot be identical with any natural property.
-According to Moore, the good is indefinable.