John Stuart Mill’s 1861 remains one of the most widely known and influential works of moral philosophy ever written. It is also a model of critical thinking – one in which Mill’s reasoning and interpretation skills are used to create a well-structured, watertight, persuasive argument for his position on core questions in ethics. The central question, for Mill, was to decide upon a valid definition of right and wrong, and reason out his moral theory from there. Laying down valid, defensible definitions is a crucial aspect of good interpretative thinking, and Mill gets his in as early as possible. Actions are good, he suggests, if they increase happiness, and bad if they reduce happiness.