On Liberty is often said to be one of, if not the most significant influences on the individualistic, tolerant culture of today. Its author, John Stuart Mill, was inspired to write this work after having his own liberty restricted all throughout his childhood by his demanding father. His father imposed a strict educational regime upon his son: Mill had read all the major Ancient Greek philosophical works by age 9 – in Ancient Greek! So he was groomed by his father, molded into a figure not of his own choosing, and he realized this at age 20, when he thought that everything about him was not a result of his own freedom, but he was almost a creation of someone else; this caused him to have a serious mental breakdown, and it took him a few years to recover from this catastrophic realization, now knowing the importance of individual liberty for our happiness.
"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"
The fundamental underlying principle of Mill's whole philosophy is described as thus: 'Over one's mind and over one's body, the individual is sovereign', and that the only instance that this can be broken is if the sovereign will of the harm of another. So, for example, it has been used to try and legitimize all drugs, as an individual should be able to choose what they do to themselves, arguing it only affects themselves; but there are problems when others rely on you: a mother for instance, would not be justified in claiming her drug-use was only affecting herself, as the effects of the drugs would impair her ability to take care of her children; so there are great difficulties in defining exactly what constitutes sovereignty.
"The only freedom which deserves the name is that of pursuing our own good in our own way, so long as we do not attempt to deprive others of theirs, or impede their efforts to obtain it"
This really only constitutes physical / material harm however; Mill was also gravely concerned about social stagnation, brought about by widespread and sheep like adherence to societal norms and beliefs, that created a world of uniformity. So he felt that invasion of personal sovereignty extended to a cultural level, where people were made to feel alienated and ridiculous for holding unorthodox beliefs, and that this certain societal coercion and pressure restricted the spiritual freedom of all of society. This concept has been reproduced in pretty much every century ever since in one form or another, most recently the 'Beat Generation' of the 1950's and 60's. On Liberty is a book outlining the benefits of being highly tolerant of differences in society, and as such is a great read when keeping in mind the tolerance of many societies today.
"No one can be a great thinker who does not recognize that as a thinker it is his first duty to follow his intellect to whatever conclusions it may lead"