Because Quirrel referred to Harry as Tom Riddle. Its possible that Quirrel put some other item which is a Horocrux of Tom Riddle in Harry's vicinity, but the diary is by far the most probable because it mirrors canon.
My interpretation of that was extremely different: that Harry got riddled when he was a baby, in Godric's Hollow.
In canon, Ginny reads the diary a lot and this enables Riddle to take her over when he wants to. When he does, she's basically a puppet: it's (fully aware) Riddle scrawling on walls and summoning basilisks, and Ginny's completely unaware of it; afterwards, Ginny is basically her normal self again, with no memory of what Riddle did while operating her body.
There's no sign in HPMOR of anything like that happening to Harry. The Harry whom QQ addres...
Two new short chapters! Since the next one is coming tomorrow and we know it'll be short, let's use one thread for both.
This is a new thread to discuss Eliezer Yudkowsky’s Harry Potter and the Methods of Rationality and anything related to it. This thread is intended for discussing chapter 105 (and chapter 106, once it comes out tomorrow). EDIT: based on Alsadius' comment about thread creation for MOR chapters, let's also use this thread for chapter 107 (and future chapters until this nears 500 comments) unless someone objects to doing so. Given that this is the final arc we're talking about, thread titles should be updated to indicate chapters covered.
There is a site dedicated to the story at hpmor.com, which is now the place to go to find the authors notes and all sorts of other goodies. AdeleneDawner has kept an archive of Author’s Notes. (This goes up to the notes for chapter 76, and is now not updating. The authors notes from chapter 77 onwards are on hpmor.com.)
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: