Gary's preference is not itself justification, rather it recognizes moral arguments, and not because it's Gary's preference, but for its own specific reasons. Saying that "Gary's preference states that X is Gary_right" is roughly the same as "Gary should_Gary X".
(This should_T terminology was discouraged by Eliezer in the sequences, perhaps since it invites incorrect moral-relativistic thinking, as if any decision problem can be assumed as own by any other, and also makes you think of ways of referring to morality, while seeing it as a black box, instead of looking inside morality. And you have to look inside even to refer to it, but won't notice that until you stop referring and try looking.)
By saying "Gary should_gary X", do you mean that "Gary would X if Gary was fully informed and had reached a state of reflective equilibrium with regard to terminal values, moral arguments, and what Gary considers to be a moral argument"?
To a first approximation, but not quite, since it might be impossible to know what is right, for any computation not to speak of a mere human, only to make right guesses.
This makes should-statements "subjectively objective"
Every well-defined question has in a sense a "subjectively objective" answer: there's "subjectivity" in the way the question has to be interpreted by an agent that takes on a task of answering it, and "objectivity" in the rules of reasoning established by such interpretation, that makes some possible answers incorrect with respect to that abstract standard.
Or, perhaps you are saying that one cannot give a concise definition of "should,"
I don't quite see how this is opposed to the other points of your comment. If you actually start unpacking the notion, you'll find that it's a very long list. Alternatively, you might try referring to that list by mentioning it, but that's a tricky task for various reasons, including the need to use morality to locate (and precisely describe the location of) the list. Perhaps we can refer to morality concisely, but it's not clear how.
(This should_T terminology was discouraged by Eliezer in the sequences, perhaps since it invites incorrect moral-relativistic thinking, as if any decision problem can be assumed as own by any other, and also makes you think of ways of referring to morality, while seeing it as a black box, instead of looking inside morality. And you have to look inside even to refer to it, but won't notice that until you stop referring and try looking.)
I had no idea what Eliezer was talking about originally until I started thinking in terms of should_T. Based on that and...
In You Provably Can't Trust Yourself, Eliezer tried to figured out why his audience didn't understand his meta-ethics sequence even after they had followed him through philosophy of language and quantum physics. Meta-ethics is my specialty, and I can't figure out what Eliezer's meta-ethical position is. And at least at this point, professionals like Robin Hanson and Toby Ord couldn't figure it out, either.
Part of the problem is that because Eliezer has gotten little value from professional philosophy, he writes about morality in a highly idiosyncratic way, using terms that would require reading hundreds of posts to understand. I might understand Eliezer's meta-ethics better if he would just cough up his positions on standard meta-ethical debates like cognitivism, motivation, the sources of normativity, moral epistemology, and so on. Nick Beckstead recently told me he thinks Eliezer's meta-ethical views are similar to those of Michael Smith, but I'm not seeing it.
If you think you can help me (and others) understand Eliezer's meta-ethical theory, please leave a comment!
Update: This comment by Richard Chappell made sense of Eliezer's meta-ethics for me.