What does "existence" have to do with anything though? Even if the real world, or morality don't "exist" in some sense, you still go on making decisions, reason about their properties. (There appear to be two useful senses of "doesn't exist": the state of some system is such that some property isn't present; or a description of a system is contradictory. These don't obviously apply here.)
The trouble is, human value might turn out to talk about complicated mathematical objects, just like mathematicians can think about (simpler kinds of) them, and it's not clear where to draw the line, at least to me while I still have too little understanding of what humane value is.
Reasoning about math objects, as opposed to just patterns of the world, seems to be analogous to reasoning about the world, as opposed to just patterns in observation. I don't believe there has to be a boundary around physics, a kind of "physical solipsism". And limitations of representation don't seem to solve the problem, as formal systems might be unable to capture particular classes of models of interest, but they can be seen as intended to elucidate properties of those models.
(There appear to be two useful senses of "doesn't exist": the state of some system is such that some property isn't present; or a description of a system is contradictory. These don't obviously apply here.)
In set theory there's a formal symbol that's read "there exists", and a pair of formal symbols that are read "there doesn't exist". Do you think these symbols should be understood in either of your two useful senses?
Update: Discussion has moved on to a new thread.
The hiatus is over with today's publication of chapter 73, and the previous thread is approaching the 500-comment threshold, so let's start a new Harry Potter and the Methods of Rationality discussion thread. This is the place to discuss Eliezer Yudkowsky's Harry Potter fanfic and anything related to it.
The first 5 discussion threads are on the main page under the harry_potter tag. Threads 6 and on (including this one) are in the discussion section using its separate tag system. Also: one, two, three, four, five, six, seven. The fanfiction.net author page is the central location for information about updates and links to HPMOR-related goodies, and AdeleneDawner has kept an archive of Author's Notes.
As a reminder, it's often useful to start your comment by indicating which chapter you are commenting on.
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically: