What does it mean when we say something is “good”? Does it mean it is pleasurable? That is true when we give to charity (assuming it is not coerced). If giving to charity is good, is goodness thus defined as giving to charity? G. E. Moore, a major twentieth century ethicist, says no, that goodness has limitless iterations which makes it indefinable, irreducible, and unanalyzable. If it is indefinable, then how are we to know what is good? Like Hume, he believes we know it when we see it, that it has an innate quality: we know it through intuition.
Having established that individual acts cannot define goodness, he criticizes the use of specific words, such as "desirable" or "pleasant", to characterize it; nor can one infer that because an act is natural it must be good or if unnatural, bad. He calls this “the naturalistic fallacy”. He also claims that when talking about goodness, the whole is greater than the sum of its parts. This he calls “the principle of organic unity”, which is discussed in the context of beautiful objects: for example, a waterfall is water continuously falling off a precipice, often with trees in the background. The objects comprising the waterfall--water, stones, trees--are not beautiful in themselves, but when assembled together by the human mind, they are. Moore asserts that aesthetic contemplation, together with the pleasure derived from social intercourse, have ethical value.
At this point you might ask, “If Moore says that goodness is indefinable, that if you try to define it by reference to specific acts you are committing the naturalistic fallacy, and that it can only be determined by intuition (which arguably is subjective), then how do we have a discussion? I’d say that’s a good question and the answer lies in a close reading of his text.
Speaking of text, any edition of Principia Ethica is acceptable. The one I own is published by Prometheus Books (1988) and is available from amazon.com (http://www.amazon.com/Principia-Ethica-Great-Books-Philosophy/dp/0879754982/ref=sr_1_1?ie=UTF8&qid=1337797721&sr=8-1) ($4.74 new, from $0.02 used). To read a free public domain copy of the work, click here (http://fair-use.org/g-e-moore/principia-ethica/).
The following resources provide additional background, analyses, and bibliographies:
Wikipedia: "G. E. Moore" (http://en.wikipedia.org/wiki/G._E._Moore), "Naturalistic Fallacy" (http://en.wikipedia.org/wiki/Naturalistic_Fallacy), "Open-question Argument" (http://en.wikipedia.org/wiki/Open-question_argument)
Stanford Encyclopedia of Philosophy: "George Edward Moore" (http://plato.stanford.edu/entries/moore/), "Moore's Moral Philosophy" (http://plato.stanford.edu/entries/moore-moral/)
Internet Encyclopedia of Philosophy: "George Edward Moore" (http://www.iep.utm.edu/moore/)
Stephen Darwall, "Moore, Normativity, and Intrinsic Value" (http://www-personal.umich.edu/~sdarwall/moore,%20normativity,%20and%20intrinsic%20value.pdf) Ethics 113 (April 2003): 468–489
Tristram McPherson, “Open Question Argument” (http://filebox.vt.edu/users/tristram/Docs/McPherson-OpenQuestion-RoutledgeEncyclopedia.pdf) forthcoming in Routledge Encyclopedia of Philosophy
Caj Strandberg, “In Defense of the Open Question Argument” (http://www.colorado.edu/philosophy/heathwood/pdf/strandberg_idoqa.pdf) The Journal of Ethics 8 (2004): 179–196
Julia Tanner, “The Naturalistic Fallacy” (http://www.richmond-philosophy.net/rjp/back_issues/rjp13_tanner.pdf) Richmond Journal of Philosophy 13 (Autumn 2006)
Jonas Olson, “G. E. Moore on Goodness and Reasons” (http://www.dartmouth.edu/~phil/docs/jolson-paper.pdf) The Australasian Journal of Philosophy 84 (December 2006): 525-534.
Alex Scott, “George Edward Moore’s Principia Ethica” (I couldn’t establish the author’s background, but this summary is more clearly written than many scholarly commentaries)
"G. E. Moore: 1873-1958" (http://faculty.fullerton.edu/mring/Moore.htm) by Professor Merrill Ring (Emeritus), Dept. of Philosophy, California State University at Fullerton
"Moore, Principia Ethica, Chapter 1" by Professor Nikko Kolodny, Dept. of Philosophy, University of California at Berkeley