What are Mill's views on freedom of thought and expression in On Liberty?

Mill holds a very passionate view of freedom of thought
and expression in On Liberty.  Mill’s ideas have been a source of
inspiration for those concerned with civil liberty and individual freedom.  Mill's
fundamental premise was concerned with the social or political order and their
relationship to individual freedom.  Mill argues that unless there is a specific risk
posed to another person, there should not be a limitation or curtailing of freedom. 
Mill was concerned much more with physical and material harm than with moral or
spiritual harm when he asserted that society might restrain the individual from harming
others.  Flying in the face of other theories he had advocated in his writing, Mill's
passionate defense of individual freedom of though and expression becomes his
fundamental driving force in the work.