An American election season seems like a bad time to sing the praises of human rationality. Candidates make promises that will never be kept yet voters somehow accept; thoughtful arguments hold no sway, while sound bites carry the day. What a comedown from the Enlightenment ideals, the faith in rationality, that inspired the founding of the republic. And it is even worse than you might think. Some things you think should be possible to figure out rationally if only you exerted yourself aren't. If you actually succeeded in living a life of reason—never voting without weighing each candidate's record carefully, never buying an appliance without consulting Consumer Reports, never begging the question, never erecting straw men, never falling into any of the other traps that flesh is heir to—you still would find yourself doing things that made no sense, not because you had failed but because reason itself is a saw blade missing a few teeth.
Throughout the 20th century scientists and mathematicians have had to accept that some things will always remain beyond the grasp of reason. In the 1930s Kurt Gödel famously showed that even in the rational universe of mathematics, for every paradox that deep thinking slaps down, new ones pop up. Economists and political theorists found similar limitations to rational rules for organizing society, and historians of science punctured the belief that scientific disputes are resolved purely by facts. The ultimate limits on reason come from quantum physics, which says that some things just happen and you can never know why.