I feel like Hofstadter was one of those people thinking really deeply about AI.

Anyone who doesn't know what I'm talking about should read 'Goedel, Escher, Bach', or 'Fluid Analogies'. I haven't read them in a long while, but I'm sure they're going to be relevant for decades, because they deal with the fundamental challenge of what it means to think. Backpropagation may be part of the puzzle, but the brain (and intelligence) is so much more than that.