This is that, in trying to write programs to simulate human intelligence, we're competing against a billion years of evolution. And that's damn hard. One counterintuitive consequence is that it's much easier to program a computer to beat Garry Kasparov at chess than to program a computer to recognize faces under varied lighting conditions. Often the hardest tasks for AI are the ones that are trivial for a five-year-old – since those are the ones that are so hardwired by evolution that we don't even think about them.
