Where the Heart of Anti-Shurtugal Rises Again.

We do get an explanation of how to throw up a mental barrier earlier in the book. You have to fill your mind with just one thing. Brom gives thinking of a brick wall as an example and Eragon later tries to think of a single one of Saphira's scales to do so. The logic being that if you're just thinking of one thing then that's all the mind reader will be able to find. He probably should have reinforced the point here again by describing what Eragon was doing to try and fail to create a mental barrier, but it is a bit of the world that is explained.

I have the next spork ready to go but I'll let this one stew for a day or tow before posting.