Humans today are often portrayed as cavemen out of time, poised to react to a lion in the grass with a suite of biases, blind spots, fallacies, and illusions. But this, Pinker a cognitive scientist and rational optimist argues, cannot be the whole picture. Hunter-gatherers--our ancestors and contemporaries--are not nervous rabbits but cerebral problem-solvers. A list of the ways in which we are stupid cannot explain how we're so smart: how we discovered the laws of nature, transformed the planet, and lengthened and enriched our lives. Indeed, if humans were fundamentally irrational, how did they discover the benchmarks for rationality against which humans fall short?