What is the theme in On Liberty by John Stuart Mill?

The author begins the book by explaining that he means 'liberty' as civil liberties. The individual in relation to the laws and customs of the land are the concern of his treatise. His reason for making this clear is because there are other ways to address the matter of 'liberty'.

He first explores what individuality is within the context of being a person who is a member of society. Then he goes on to describe how the behavior of an individual relates to the laws of a given land, in this case England. He differentiates between the conscience and the totality of an individual.