Ten years elapsed between the publication of "Political Economy" (see Vol. XIV, p. 294) and the "Essay on Liberty," Mill in the meantime (1851) having married Mrs. John Taylor, a lady who exercised no small influence on his philosophical position. The seven years of his married life saw little or nothing from his pen. The "Essay on Liberty," in many respects the most carefully prepared of all his books, appeared in 1859, the year following the death of his wife, in collaboration with whom it was thought out and partly written. The treatise goes naturally with that on "Utilitarianism." Both are succinct and incisive in their reasoning, and both are grounded on similar sociological principles. Perhaps the primary problem of politics in all ages has been the reconciliation of individual and social interests; and at the present day, when the problem appears to be particularly troublesome, Mill's view of the situation is of especial value. In recent time, legislation has certainly tended to become more socialistic, and the doctrine of individual liberty promulgated in this "Essay" has a most interesting relevancy to modern social movements. _I.--Liberty of Thought and Discussion_ Protection against popular government is as indispensable as protection against political despotism. The people may desire to oppress a part of their number, and precautions are needed against this as against any other abuse of power. So much will be readily granted by most, and yet no attempt has been made to find the fitting adjustment between individual independence and social control. The object of this essay is to assert the simple principle that the sole end for which mankind are warranted, individually or collectively, in interfering with the liberty of action of any of their number is self-protection--that the only purpose for which power can be rightfully exercised over any member of a civilised community, against his will, is to prevent harm to others, either by his action or inaction. The only part of the conduct of anyone for which he is amenable to society is that which concerns others. In the part which merely concerns himself, his independence is, of right, absolute. Over himself, over his own body and mind, the individual is sovereign. This principle requires, firstly, liberty of conscience in the most comprehensive sense, liberty of thought and feeling; absolute freedom of opinion and sentiment on all subjects, practical or speculative, scientific, moral, or theological--the liberty even of publishing and expressing opinions. Secondly, the principle requires liberty of tastes and pursuits; of framing the plan of our life to suit our own character; of doing as we like, so long as we do not harm our fellow-creatures. Thirdly, the principle requires liberty of combination among individuals for any purpose not involving harm to others, provided the persons are of full age and not forced or deceived.