The Subjection of Women (1869) by John Stuart Mill contributed to liberal political theory by extending and developing the ideology of individualism to women. Mills thesis was that "the legal subordination of one sex to the other is wrong in itself and now one of the chief hindrances to human improvement; and it ought to be replaced by a principle of perfect equality." His feminist analysis was radical in nineteenth-century England, but it provided the roots of liberal feminist theory in the twentieth-century United States.