It's the chilling plot line to every science fiction movie about robots in the future: Once they start thinking for themselves, humanity is doomed. Think of the HAL 9000 in "2001: A Space Odyssey," or the replicants in "Blade Runner," or the hosts in "Westworld." These days the Pentagon is doing a lot of thinking about the nascent scientific field of artificial intelligence, also known as "machine learning," developing computer algorithms that will allow cars to drive themselves, robots to perform surgery, and even weapons to kill autonomously. The race to master artificial intelligence is the No. 1 priority of the Defense Advanced Research Projects Agency, the tiny organization with just over 200 workers that was instrumental in developing stealth technology, high precision weapons, and the Internet. "In reality, over about the last 50 years, DARPA and its research partners have led the way to establishing the field of artificial intelligence. We are not new to this game," said DARPA Director Steven Walker at the agency's 60th anniversary symposium in September.