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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 philosopher 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 civilised community, against his will, is to prevent harm to others. His own good, either physical or moral, is not a 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.
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 obligations. 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.
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 to the right of sedition.
The United States Libertarian Party includes a version of the harm principle as part of its official party platform. It states:
Criminal laws should be limited in their application to violations of the rights of others through force or fraud, or to deliberate actions that place others involuntarily at significant risk of harm. Therefore, we favor the repeal of all laws creating “crimes” without victims . . .
Scholars[who?] have argued that the harm principle doesn't provide a narrow scope of which actions count as harmful towards oneself or the population and it cannot be used to determine whether people can be punished for their actions by the state. A state can determine whether an action is punishable by determining what harm the action causes. If a morally unjust action occurs but leaves no indisputable form of harm, there is no justification for the state to act and punish the perpetrator for their actions. The harm principle has an ambiguous definition of what harm specifically is and what justifies a state to intervene.
Scholars[who?] have also said that the harm principle does not specify on whether the state is justified with intervention tactics. This ambiguity can lead to a state defining what counts as a harmful self-regarding action at their own discretion. This freedom might allow for an individual's own liberty and rights to be in danger. It would not be plausible for a state to intervene with an action that will negatively affect the population more than an individual. The harm principle scope of usage has been described as too wide to directly follow and implement possible punishment by a state.