Book excerpt: In this book, Keith Stanovich attempts to resolve the Great Rationality Debate in cognitive science-the debate about how much irrationality to ascribe to human cognition. Stanovich shows how the insights of dual-process theory and evolutionary psychology can be combined to explain why humans are sometimes irrational even though they possess cognitive machinery of remarkable adaptiveness. Using a unique individual differences approach, Stanovich shows that to fully characterize differences in rational thinking, the traditional System 2 of dual-process theory must be partitioned into the reflective mind and the algorithmic mind. Using a new tripartite model of mind, Stanovich shows how rationality is a more encompassing construct than intelligence-when both are properly defined-and that IQ tests fail to assess individual differences in rational thought. Stanovich discusses the types of thinking processes that would be measured in an assessment of rational thinking.