I'm currently reading Moore's Principia Ethica, a most

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I'm currently reading Moore's Principia Ethica, a most interesting treatise on (meta)ethics. Now, on that topic: does anyone have a satisfactory definition of 'good' (or 'goodness', if you will) or must we accept that, in the words of Moore, "good is a simple notion, just as 'yellow' is a simple notion; that, just as you cannot, by any manner of means, explain to any one who does not already know it, what yellow is, so you cannot explain what good is".

In other words, what do you think of his Open Question Argument:

P1. Suppose that the predicate 'good' is synonymous with some other predicate N (e.g., 'pleasurable').
P2. 'X has the property N' will mean 'X is good'.
>C1. Anybody who would ask whether an X with property N is good, would ipso facto betray conceptual confusion. (S)he is unaware of what 'good means (symmetry of identity, P2).
P3. Howevery, for every N it is always an open question whether an X with property N is good. It is a meaningful question that does not demonstrate conceptual confusion.
P4. If for every N it is always an open question whether an X with property N is good, then 'N' cannot be synonymous with 'good'.
>C2. 'N' cannot be synonymous with 'good' (modus ponens, P3, P4).
P5. If N cannot be synonymous with 'good', then only 'good' can be synonymous with 'good'; therefore, good is a simple (primitive) concept and cannot be defined.
>C3. Only good can be synonymous with 'good'; therefore, good is a simple (primitive) concept and cannot be defined (modus ponens, C2, P5).

Was Plato right after all?

(Spookbusters not allowed)