The Full quote

Mill, however, is not quite a free speech absolutist, though he does come much closer than many other philosophers. He proposes “one simple principle” to determine when one’s liberty, including one’s right to free speech, may be infringed. The principle is known as the harm principle, and he argues that it provides the only justification for interfering with the right to free speech. The harm principle states that the sole end for which individuals or society can put limits on a person’s liberty (including free speech) is self-protection. Put another way, the harm principle says that the only justifiable interference with a person’s liberty is to prevent the direct, serious harm of another person.