What is intelligence? Can it be increased by teaching? If so, how, and what difference would an increase make? Before we can answer these questions, we need to clarify them. Jonathan Baron argues that when we do so we find that intelligence has much to do with rational thinking, and that the skills involved in rational thinking are in fact teachable, at least to some extent. Rationality and Intelligence develops and justifies a prescriptive theory of rational thinking in terms of utility theory and the theory of rational life plans. The prescriptive theory, buttressed by other assumptions, suggests that people generally think too little and in a way that is insufficiently critical of the initial possibilities that occur to them. However these biases can be - and sometimes are - corrected by education.