To write about Mill is to write about education. In the opening paragraph of his Autobiography Mill explains:

I have thought that in an age in which education, and its improvement, are the subject of more, if not of profounder study than at any former period of English history, it may be useful that there should be some record of an education which was unusual and remarkable, and which, whatever else it may have done, has proved how much more than is commonly supposed may be taught, and well taught, in those early years which, in the common modes of what is called instruction, are little better than wasted. (Mill, 1874a, p. 25)