I see from at least some of the comments that some defenders of physicalism here are struggling with this argument. Two prominent philosophers have made this point. Atheist Thomas Nagel in "Mind and Cosmos" and theist philosopher Alvin Plantinga explores it in great detail in "Where the Conflict Really Lies." Even Brian Greene, perhaps the best defender of physicalism, in "Until the End of Time" recognizes that accepting that nothing exists outside the random accumulation of particles cannot lead to any confidence in our rational thoughts. Even Darwin expressed "horrid doubt" about his convictions could have any meaning if they derived from monkey brains.