William Bernstein, a neurologist turned financial theorist and investment advisor, organizes his sophisticated investment framework around four essential pillars: the theory of investing, the history of markets, the psychology of investor behavior, and the business of the investment industry. Together, these four lenses provide a complete picture of why passive, low-cost, diversified investing consistently outperforms the alternatives over long time horizons. On theory, Bernstein explains the relationship between risk and expected return, the mathematics of diversification, and the evidence for and against market efficiency. On history, he shows how markets have rewarded patient investors across two centuries of crises, wars, and economic upheaval. On psychology, he catalogues the cognitive biases and emotional reactions that cause investors to consistently buy high and sell low. And on the investment business, he provides a frank assessment of how brokers, active fund managers, and financial media profit by encouraging the kind of activity and complexity that reduces investor returns. Dense with data but written for the educated general reader, this is one of the most rigorous and complete guides to investment theory available outside a graduate finance textbook.