Jeremy Bentham, 1748-1832, is seen as the founder of Utilitarianism. As well as developing the theory, he also showed how it worked in practice, bringing about many powerful social reforms. He argued for equal rights for women, the right to divorce, an end to slavery etc. Some credit him with laying the foundations for the development of a welfare state.
Bentham's moral philosophy looked at what we desire - pleasure - and reasoned that the morally right course of action must be the one that brings the most pleasure and greatest absence of pain. His student, John Stuart Mill (1806-1873), was impressed with Bentham's theory and his social reforms, but Mill saw weaknesses in the theory and adapted it to make it stronger.
Although Bentham and Mill may seem like the philosophers of the common man, concerned with ordinary people and bringing ethics down to a practical level that benefits us all, they were in fact quite remarkable. Bentham went to Oxford aged 12, got his degree at 15 and his Masters aged 18. He was called to the bar aged 21.
Mill was another child prodigy. He learnt Greek at 3, studied Plato aged 8 when he also began learning Latin, studying Euclid and algebra, and teaching younger children. Aged 12, Mill began a thorough study of scholistic logic; at 13 political economy. By 21, his ferocious studying brought on a nervous breakdown.
Like Bentham, Mill was concerned with improving society. He became a politician, the first person in parliament to call for women to be given the vote. However it was through his writings and his development of Bentham's theory of utilitarianism that he is best known, and their ethical theory, repeatedly adapted but essentially unchanged, is probably the most popular and commonplace among ethicists and ordinary people in the Western world today.