The Greatest Happiness Principle is the central idea of 19th century English philosopher John Stuart Mill’s brand of utilitarianism. Utilitarianism is a branch of philosophy that determines the morality of a choice based on the total outcomes of each decision. For Mill, the morally correct choice will always benefit the largest number of people. It is a belief that has some serious implications for how society should be structured and how people live their lives.

In Mill’s own words, the Greatest Happiness Principle

holds that actions are right in proportion as they tend to promote happiness, wrong as they tend to produce the reverse of happiness.

Mill further defines happiness as “pleasure and the absence of pain.” Mill also proposed that this is the natural state of humanity by arguing that happiness is the only thing people truly seek. As a result, happiness is the sole basis of human morality. Mill did provide some caveats to this theory by classifying happiness into "base" and higher categories. He claims that sometimes the "base" happiness, such as sexual fulfillment or gluttony, should be tossed aside in favor of higher, more valuable forms of happiness. In this way, he avoids accusations of hedonism.

John Stuart Mill's idea that morally good decisions always promote the greatest happiness necessarily defines evil as that which causes pain or opposes pleasure for the majority. This has serious logical ramifications in the real world and has led to many detractors of and critiques of Mill’s work.