"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." (John Stuart Mill, On Liberty)
In other word, Mill is saying that an individual can do anything as long as his/her action does not harm others. That 'anything' includes anything that he/she does on his/herself.
A postfoundationalist like 'lim peh'(Hokkien: myself) will say that one cannot make a sure judgment on the (butterfly) effects of one's action have on others, especially within our myriad existent. And this is our inability. Hence that means one's action cannot be justified by our inability to conceive the effect.
Mill's suggestion is only true if the individual has absolute certainty over the effects of one's action. This presupposition is not unusual among pre-postmodern (not necessary 'modern') discourse.
Given our current awareness (thanks to postmodern thinkers) of context and the essentially interdependent socio-political forces operating in society, we can hardly and simply be 'independent'.
Let me draw an illustration from current economic crisis.
It started with Mr. and Mrs. Individuals who lives at a certain town in America. They have some saving and they aspire to enlarge their savings by investing in property. So they took up a loan from a local bank for the mortgage of the property.
The local bank mortgage manager, typically by the same name Ms. Individual, approved the loan of the couple. "What's wrong by loaning out money to those who need it to acquire a property? It helps them, the company that I'm working in, and a good appraisal for myself at the end of the year," thought Ms. Individual.
The mortgage manager's superior is happy with her excelling in her job. The superior feedback to the director of the company. The director is happy with the job and told his congressman friends. The congressmen delighted over the system of giving loan to help Mr. and Mrs. Individuals to acquire asset.
Such generousity spread through the layers of the society, from the congress to the director, to the superior, to the mortgage manager, to Mr. and Mrs. Individuals.
Little did everyone along this line of 'generousity' take note that there are millions of others who are like Mr. and Mrs. Individuals, hundred of thousands of others who are like Ms. Individual, thousands of other who are like Ms. Individual's superior, and hundreds of others who are like the director of the company doing the same thing.
All of these people acted in their own independent way, without intending harm on any one. Realizing Mill's dream.
And the rest is a long chain of effects. First we had the property bubble in America for about 2 years. Then followed by financial crisis in the states. Then Lehman Brothers filed for bankruptcy on 15 September 2008 at the U.S. Bankruptcy Court, Southern District of New York. Then just approximately two months later the strongest bank in South East Asia, the DBS Bank located at the Shenton Way reported that it has retrenched 900 of its staffs.
And unknowingly to Mr. and Mrs. Individuals in the U.S, there is also an expatriate by the same name lost his job at DBS.
The example is just to modestly illustrate that we just could not be certain of the (butterfly) effects that an individual's sovereign act over him/herself have on others. Therefore Mill's suggestion that an individual's liberty should be measured and limited by its effects is invalid and unhelpful, especially in political discourse.
Simply put, Mill said that our ethics should hinge on the no-harm-on-others principle. He presupposed that we are able to conceive the effects of our individual's actions.
But I say to Mill that you can't be sure of the limit and degree of the harm, and you may not see its harmful effect at all. Hence your suggestion is not justifiable. Shouldn't that hold us back rather than opening up the gates of free-action?