John Stuart Mill was a prominent Nineteenth Century bourgeois philosopher and economist, who did, however, have some sympathy for the new political doctrine of socialism which developed during his lifetime. He is best known as a champion of UTILITARIANISM, which he imbibed from his father James and his father's friend JEREMY BENTHAM.
While Mill defended a more nuanced version of utilitarianism than did his father and Bentham, he nevertheless continued Bentham's perversion of the central notion of utility (by focusing on happiness and pain, rather than on the more direct concept of usefulness). But Mill did reject Bentham's crude hedonistic calculus, and distinguished between the qualities of various sorts of happiness as well as the quantities of happiness. Thus, while the following is perhaps just a wee bit unfair, it is too funny to neglect!
There once was a man named John Mill
Utility gave him a thrill.
Calculating, he would
Declare murder good
(Depending on whom you will kill).
Mill also wrote a massive work on logic and related topics, from an empiricist point of view. Among the many doubts that have been raised about this approach, there is:
According to John Stuart Mill,
Better check whether 2+2 still
Make four. They did today,
But as for tomorrow—who can say?
And then there is Mill's work on political economy, which became the leading bourgeois textbook on the subject until the marginalists came on the scene and removed any last semblance of compassion and social justice from the dominant economic theories defending capitalism.
John Stuart Mill,
By a mighty effort of will,
Overcame his natural bonhomie
And wrote "Principles of Political Economy".
—Edmund Clerihew Bentley
Lest anyone think this last item is off topic, Dean Zimmerman has pointed out that the full title of Mill's book is: "Principles of Political Economy, With some of their Applications to Social Philosophy".