But is the justification for its global applicability that "if everyone lived by that rule, average happiness would be maximized"?
Well, not, that's not Kant's justification!
That (or any other such consideration) itself is not a mandatory goal, but a chosen one.
Why would a rational agent choose unhappiness?
If you find yourself to be the worshipped god-king in some ancient Mesopotanian culture, there may be many more effective ways to make yourself happy, other than the Categorical Imperative.
Yes, but that wouldn't count as ethics. You wouldn't want a Universal Law that one guy gets the harem, and everyone else is a slave, because you wouldn't want to be a slave, and you probably would be. This is brought out in Rawls' version of Kantian ethics: you pretend to yourself that you are behind a veil that prevents you knowing what role in society you are going to have, and choose rules that you would want to have if you were to enter society at random.
My argument against moral realism and assorted is that if you had an axiomatic system from which it followed that strawberry is the best flavor of ice cream, but other agents which are just as intelligent with just as much optimizing power could use different axiomatic systems leading to different conclusions,
You don't have object-level stuff like ice cream or paperclips in your axioms (maxims), you have abstract stuff, like the Categorical Imperative. You then arrive at object level ethics by plugging in details of actual circumstances and values. These will vary, but not in an arbitrary way, as is the disadvantage of anything-goes relativism.
how could one such system possibly be taken to be globally correct and compelling-to-adopt across agents with different goals?
The idea is that things like the CI have rational appeal.
Once you've taken a rational agent apart and know its goals and, as a component, its ethical subroutines, there is no further "core spark" which really yearns to adopt the Categorical Imperative.
Rational agents will converge on a number of things because they are rational. None of them will think 2+2-=5.
Yes, but that wouldn't count as ethics. You wouldn't want a Universal Law that one guy gets the harem, and everyone else is a slave, because you wouldn't want to be a slave, and you probably would be.
If there are a lot of similar agents in similar positions, Kantian ethics works, no matter what their goals. For example, theft may appear to have positive expected value - assuming you're selfish - but it has positive expected value for lots of people, and if they all stole the economy would collapse.
OTOH, if you are in an unusual position, the Categorical...
A few notes about the site mechanics
A few notes about the community
If English is not your first language, don't let that make you afraid to post or comment. You can get English help on Discussion- or Main-level posts by sending a PM to one of the following users (use the "send message" link on the upper right of their user page). Either put the text of the post in the PM, or just say that you'd like English help and you'll get a response with an email address.
* Normal_Anomaly
* Randaly
* shokwave
* Barry Cotter
A note for theists: you will find the Less Wrong community to be predominantly atheist, though not completely so, and most of us are genuinely respectful of religious people who keep the usual community norms. It's worth saying that we might think religion is off-topic in some places where you think it's on-topic, so be thoughtful about where and how you start explicitly talking about it; some of us are happy to talk about religion, some of us aren't interested. Bear in mind that many of us really, truly have given full consideration to theistic claims and found them to be false, so starting with the most common arguments is pretty likely just to annoy people. Anyhow, it's absolutely OK to mention that you're religious in your welcome post and to invite a discussion there.
A list of some posts that are pretty awesome
I recommend the major sequences to everybody, but I realize how daunting they look at first. So for purposes of immediate gratification, the following posts are particularly interesting/illuminating/provocative and don't require any previous reading:
More suggestions are welcome! Or just check out the top-rated posts from the history of Less Wrong. Most posts at +50 or more are well worth your time.
Welcome to Less Wrong, and we look forward to hearing from you throughout the site!
Note from orthonormal: MBlume and other contributors wrote the original version of this welcome post, and I've edited it a fair bit. If there's anything I should add or update on this post (especially broken links), please send me a private message—I may not notice a comment on the post. Finally, once this gets past 500 comments, anyone is welcome to copy and edit this intro to start the next welcome thread.