The reason I take second-order logic seriously is that it lets me pin down a single mathematical referent that I'm comparing to the realities of space and time.
How can you say this after having read this thread?
If you believe in second-order model theory, then you believe in set theory. (However, by limiting it to second order over the natural numbers, without going on to third order, you are not obligated to believe in uncountable ordinals.)
ETA: It is very imprecise to compare second-order model theory and set theory like this. Already model theory is set theory, of course, albeit (potentially, not in practice) set theory without power sets. I should just leave the model theory out of it and say:
If you believe in second-order logic, then you believe in set theory. (However, ….)
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: