On Liberty (1859) was initially intended to be a short essay, but expanded into a much more substantial work espousing Mill's ethical system of utilitarianism to society and the state.
The grand, leading principle, towards which every argument unfolded in these pages directly converges, is the absolute and essential importance of human development in its richest diversity. —Wilhelm von Humboldt: Sphere and Duties of Government
TO the beloved and deplored memory of her who was the inspirer, and in part the author, of all that is best in my writings—the friend and wife whose exalted sense of truth and right was my strongest incitement, and whose approbation was my chief reward—I dedicate this volume. Like all that I have written for many years, it belongs as much to her as to me; but the work as it stands has had, in a very insufficient degree, the inestimable advantage of her revision; some of the most important portions having been reserved for a more careful re-examination, which they are now never destined to receive. Were I but capable of interpreting to the world one half the great thoughts and noble feelings which are buried in her grave, I should be the medium of a greater benefit to it, than is ever likely to arise from anything that I can write, unprompted and unassisted by her all but unrivalled wisdom.