John Stuart Mill is most famous for his work on the ethical theory of utilitarianism, based upon earlier work by Jeremy Bentham. Mill argued that the morality of actions should be judged based on how well they foster the greatest possible happiness for the greatest number of people.
John Stuart Mill was unusual in that his parents did not raise him in a religious environment or to believe religious doctrines; he was thus never a Christian and was able to approach the claims of Christianity from a more independent, objective perspective.
In his 1887 book A System of Logic, John Stuart Mill made a crucial contribution to the development of modern science: the importance of control groups in scientific experiments. Even in the modern era it remains difficult to get people to understand the necessity of testing the presence of a cause against the absence of a cause.