As regards originality, it has of course no other than that which every thoughtful mind gives to its own mode of conceiving and expressing truths which are common property. The leading thought of the book is one which though in many ages confined to insulated thinkers, mankind have probably at no time since the beginning of civilization been entirely without. To speak only of the last few generations, it is distinctly contained in the vein of important thought respecting education and culture, spread through the European mind by the labours and genius of Pestalozzi. The unqualified championship of it by Wilhelm von Humboldt is referred to in the book; but he by no means stood alone in his own country. During the early part of the present century the doctrine of the rights of individuality, and the claim of the moral nature to develop itself in its own way, was pushed by a whole school of German authors even to exaggeration; and the writings of Goethe, the most celebrated of all German authors, though not belonging to that or to any other school, are penetrated throughout by views of morals and of conduct in life, often in my opinion not defensible, but which are incessantly seeking whatever defence they admit of in the theory of the right and duty of self-development. In our own country, before the book “On Liberty” was written, the doctrine of Individuality had been enthusiastically asserted, in a style of vigorous declamation sometimes reminding one of Fichte, by Mr. William Maccall, in a series of writings of which the most elaborate is entitled “Elements of Individualism:” [sic] and a remarkable American, Mr. Warren, had framed a System of Society, on the foundation of “the Sovereignty of the Individual,” had obtained a number of followers, and had actually commenced the formation of a Village Community (whether it now exists I know not), which, though bearing a superficial resemblance to some of the projects of Socialists, is diametrically opposite to them in principle, since it recognizes no authority whatever in Society over the individual, except to enforce equal freedom of development for all individualities. As the book which bears my name claimed no originality for any of its doctrines, and was not intended to write their history, the only author who had preceded me in their assertion, of whom I thought it appropriate to say anything, was Humboldt, who furnished the motto to the work; although in one passage I borrowed from the Warrenites their phrase, the sovereignty of the individual. It is hardly necessary here to remark that there are abundant differences in detail, between the conception of the doctrine by any of the predecessors I have mentioned, and that set forth in the book.
Note from KBJ: The motto of which Mill speaks is as follows: "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." The motto comes from Humboldt's book The Sphere and Duties of Government (1791). Here is the 1854 English translation.