Moore's Principia Ethica is a central text in twentieth-century meta-ethics. According to the familiar history of the subject, the story of much of twentieth-century meta-ethics can be understood as a series of reactions to this book. In this book Moore argues for non-naturalistic intuitionism. He argues that moral properties are an irreducible part of reality, and that they are sui generis. And he argues that we can acquire knowledge of these sui generis moral properties only through intuition.
The first chapter includes Moore's famous Open Question Argument, his argument that intrinsic goodness is a simple, unanalyzable, non-natural property. There appear to be two strands of the OQA; both of them appeal to our linguistic intuitions. The first focuses on our intuitions about whether certain claims about intrinsic goodness are tautological. Borrowing Moore's own example, suppose someone tries to define 'good' as 'what is pleasant'. All competent users of the language can see that this definition must fail. How? They simply need to ask themselves if "the good is what is pleasant" has the same meaning as "the pleasant is what is pleasant," for these two sentences would be synonymous if 'good' could be correctly defined 'what is pleasant.' And, Moore claims, these sentences clearly aren't synonymous: the claim that "the good is what is pleasant" is not a tautology like "the pleasant is pleasant." This shows that 'good' and 'what is pleasant' have different meanings. Furthermore, Moore argues that thinking about other examples will show that, in principle, we could develop that a structurally similar argument against any other attempted definition of 'good'.
The second strand of the argument draws on our intuitions about what is and is not an open question. If 'what is pleasant' is a correct definition of 'good', then the following should not be an open question: Is what is pleasant good? For, if the proposed definition is correct, then it is true by definition that this what is pleasant is good. But, Moore asserts, competent users of the language don't think it's true by definition that what is pleasant is good; they do, and should, regard this question as an open one. It might be true that what is pleasant is good--but, importantly, it is not true by definition. Again, Moore argues that a similar argument will show that any other attempted definition of 'good' fails.
The conclusions Moore draws from the OQA are that the term 'good' is indefinable, and that goodness is a simple and unanalyzable property. It is somewhat unclear how Moore thinks he can draw any metaphysical conclusion from this argument, but he seems to have reasoned in the following way. If goodness were identical to some natural or metaphysical property, it would be definable in terms of some natural or metaphysical predicate. The OQA shows that it is not definable in this way, and so it shows that goodness is not identical to a property of either of these types. Moore also provides a label for the error made by those who don't understand the nature of intrinsic goodness: they commit "the naturalistic fallacy." The naturalistic fallacy is committed by anyone who identifies intrinsic goodness with some other property, and Moore claims that nearly all previous ethicists have committed this fallacy.
The Preface involves a very brief sketch of Moore's epistemological views, views that he apparently thinks follow from his conclusions about the nature of intrinsic goodness. Moore claims that knowledge of intrinsic goodness can be acquired only through intuitions. There is no non-intuitional evidence for propositions about the intrinsic goodness of things, and they cannot be deduced from any other propositions. How, then, do we come to know these things? We think very carefully about them, ensure that we don't confuse these propositions with other, and then we have an intuition of the proposition's truth or we don't.
The final two chapters are concerned with issues in normative ethics. The substantive moral theory he defends is a form of act-consequentialism. According to Moore, in all situations, the action one ought to perform is the action that will result in the greatest net amount of intrinsic goodness in the world. In the fifth chapter, he discusses what practical conclusions one can draw from this account of right action. He argues for a fairly extreme form of moral conservatism according to which we have good reason to act on the principles constituting conventional morality almost all the time. For we simply don't know enough about the consequences of the possible actions available to us to discover which of them will lead to the greatest amount of net intrinsic goodness. Given our ignorance of these matters, what we ought to do is follow most of the common moral rules in our community. (Moore does allow for exceptions in a limited number of cases, however.)
The sixth and final chapter includes Moore's discussion of which things are intrinsically good. He argues that two things, aesthetic enjoyment and personal affection, are the most intrinsically good of intrinsically good things.
For the most part, this isn't an exciting book. It's filled with the sort of philosophizing one expects from Moore: close reading, lots of subtle distinctions, and patient and careful analysis. His work is slow and painstaking--that's a given. And, to be honest, this book is occasionally dull. But Moore's scrupulous attention to detail doesn't preclude insight and interesting ideas. Nor does it bury them in pages and pages of tedious nit-picking and quibbling.
For students of meta-ethics and anyone interested in the history of twentieth-century ethical thought, this is mandatory reading.
Concerning the revised edition of Principia, edited by Thomas Baldwin. The buyer should know that the text of Principia is unedited in this edition. However, there are reasons to prefer this edition to cheaper alternatives. First, and most importantly, this edition includes the so-called "Preface to the Second Edition," which consists of Moore's later misgivings about the first chapter of Principia. (Moore never published this preface in his lifetime.) He doesn't repudiate his substantial conclusions there, but he raises issues about the statement of those conclusions and the arguments he provided for them. If you're interested in serious study of Principia, knowledge of this preface is crucially important. Second, this edition includes Moore's paper "The Conception of Intrinsic Value," which helps to clarify his views about the nature of intrinsic goodness. Third, Baldwin includes a helpful editor's introduction for this edition.