John Stuart Mill was an English philosopher, political economist and civil servant. One of the most influential thinkers in the history of liberalism, he contributed widely to social theory, political theory and political economy. He has been called "the most influential English-speaking philosopher of the nineteenth century." Mill's conception of liberty justified the freedom of the individual in opposition to unlimited state control. Mill expresses his view on freedom by illustrating how an individual's drive to better their station, and for self-improvement, is the sole source of true freedom. Only when an individual is able to attain such improvements, without impeding others in their own efforts to do the same, can true freedom prevail. Mill's linking of freedom and self-improvement has inspired many. By establishing that individual efforts to excel have worth, Mill was able to show how they should achieve self-improvement without harming others, or society at large. Among his philosophical achievements, he was a proponent of utilitarianism, an ethical theory developed by Jeremy Bentham, and he worked on the theory of the scientific method. Mill was also a Member of Parliament belonging to the Liberal Party.