Shane, I meant that AIXI is the last difficult topic.

AIXI itself is a deranged god, but that's a separate story. I'm very fond of Hutter's work, I just don't think it means what Hutter seems to think it means. AIXI draws the line of demarcation between problems you can solve using known math and infinite computing power, and problems that are essentially structural in nature. I regard this as an important line of demarcation!

It's also the first AGI specification drawn in sufficient detail that you can really nail down what goes wrong - most AGI wannabes will just say, "Oh, my AI wouldn't do that" because it's all magical anyway.