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The harm principle holds that the actions of individuals should only be limited to prevent harm to other individuals. John Stuart Mill articulated this principle in On Liberty, where he argued that, "The only purpose for which power can be rightfully exercised over any member of a civilized community, against his will, is to prevent harm to others." An equivalent was earlier stated in France's Declaration of the Rights of Man and of the Citizen of 1789 as, "Liberty consists in the freedom to do everything which injures no one else; hence the exercise of the natural rights of each man has no limits except those which assure to the other members of the society the enjoyment of the same rights. These limits can only be determined by law."
The belief "that no one should be forcibly prevented from acting in any way he chooses provided his acts are not invasive of the free acts of others" has become one of the basic principles of libertarian politics. The harm principle was first fully articulated by the English thinker John Stuart Mill (1806–1873) in the first chapter of On Liberty (1859), where he argued that:
The object of this Essay is to assert one very simple principle, as entitled to govern absolutely the dealings of society with the individual in the way of compulsion and control, whether the means used be physical force in the form of legal penalties, or the moral coercion of public opinion. That principle is, 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 civilized community, against his will, is to prevent harm to others. His own good, either physical or moral, is not sufficient warrant. He cannot rightfully be compelled to do or forbear because it will be better for him to do so, because it will make him happier, because, in the opinion of others, to do so would be wise, or even right... 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.
Mill sees harm and wrongdoing as synonymous. "Harmless wrongdoing" is redundant. Even if a self-regarding action results in harm to oneself, it is still beyond the sphere of justifiable state coercion.
Harm itself is not a non-moral concept. The infliction of harm upon another person is what makes an action wrong.
Harm can also result from a failure to meet an obligation. Morality generates obligation. Duty may be exacted from a person in the same way as a debt, and it is part of the notion of duty that a person may be rightfully compelled to fulfill it.
The offense principle 
Mill's harm principle is distinct from the offense principle. The basis of comparison is that in some cases, psychological or social harm may be comparable to physical harm. The difference is based on the assumption that offense may cause harm, but does not necessarily cause harm. An offense meets the harm principle only if it is a wrong and also causes harm.
The ethical question as to what extent there should be constraints on free speech is often grounded in both the harm principle and the offense principle. If the exercise of free speech can be causally linked to violence or similar physical harm, it is constrained under the harm principle. Free speech actions such as burning a flag or holding controversial rallies usually fall under the offense principle instead, based on the corresponding question of what constitutes harm (or, alternately, weighing harm caused by limiting a freedom vs harm caused by the exercise of that freedom).
In the abstract, criminalizing all wrongs that cause harm makes the offense principle almost synonymous with the harm principle. However, the definition of what constitutes a wrong may change over time—such that harm may be caused by actions that were not considered wrongs at the time, and vice versa. 
Broader definitions of harm 
In the same essay, Mill further explains the principle as a function of two maxims:
The maxims are, first, that the individual is not accountable to society for his actions, in so far as these concern the interests of no person but himself. Advice, instruction, persuasion, and avoidance by other people, if thought necessary by them for their own good, are the only measures by which society can justifiably express its dislike or disapprobation of his conduct. Secondly, that for such actions as are prejudicial to the interests of others, the individual is accountable, and may be subjected either to social or to legal punishments, if society is of opinion that the one or the other is requisite for its protection. (LV2)
The second of these maxims has become known as the social authority principle.
However, the second maxim also opens the question of broader definitions of harm, up to and including harm to the society. The concept of harm is not limited to harm to another individual but can be harm to individuals plurally, without specific definition of those individuals.
This is an important principle for the purpose of determining harm that only manifests gradually over time—such that the resulting harm can be anticipated, but does not yet exist at the time that the action causing harm was taken. It also applies to other issues—which range from the right of an entity to discharge broadly polluting waste on private property, to broad questions of licensing, and even to the right of sedition.
See also 
- Baselines, at Legal Theory Blog.
- "Freedom of Speech". Stanford Encyclopedia of Philosophy. 17 April 2008. Retrieved 29 May 2011.
- Ronald Hamowy, The encyclopaedia of libertarianism, Sage, 2008, p. xxi, ISBN
- John Stuart Mill (1859). On Liberty. Oxford University. pp. 21–22. Retrieved 2008-02-27.
- Menezes Oliveira, Jorge. "Harm and Offence in Mill’s Conception of Liberty". University of Oxford. Retrieved 6 Jun. 2012.
- Cohen-Almagor, Raphael (1993). Harm Principle, Offense Principle, and the Skokie Affair 41 (3). Political Studies. Retrieved 6 Jun. 2012.
- Simester, A.P.; von Hirsch, Andrew (2002). Rethinking the Offense Principle 8. Legal Theory. Retrieved 6 Jun. 2012.