The harm principle is a foundation stone of liberal thought. (I mean "liberal" in the classical, libertarian sense, not in the modern, statist sense.) John Stuart Mill articulated the harm principle in Chapter 1 of On Liberty:
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 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 opinions of others, to do so would be wise, or even right. These are good reasons for remonstrating with him, or reasoning with him, or persuading him, or entreating him, but not for compelling him, or visiting him with any evil, in case he do otherwise. To justify that, the conduct from which it is desired to deter him must be calculated to produce evil to some one else. The only part of the conduct of any one, 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.
Many proponents of the harm principle read it narrowly, as if the only harm that one may do to another is immediate or predictable (as in the case of pollution, for example). But there is more to liberty than allowing everyone to do his or her "own thing" as long as it doesn't result in immediate or predictable harm to others. We must take account of the harm that might result in the longer run from actions that are likely to strain and sunder the bonds of trust that make it possible for a people to coexist civilly. It is those bonds of trust -- forged by shared customs and moral principles -- that enable the members of society to pursue happiness with little or no fear of -- or the need to prepare for and defend against -- predations by their fellows.
It is from this deeper reading of the harm principle that I have come, in the past few years, to reject the dominant "almost anything goes" strain of libertarianism that reflexively embraces abortion and same-sex marriage, among other anti-social practices. To those of you who wish to understand my seemingly paradoxical stance on such issues -- or who entertain doubts about the wisdom of the "almost anything goes" orthodoxy -- I commend these posts (and those linked therein):
All related posts (through April 28) are collected here, under the heading "Self-Ownership."