Liberty principle (harm principle): one may justifiably limit a person"s freedom of action only if they threaten harm to another. Condition: applicable to any member of a civilized community . Argument: liberty is only valuable under certain conditions, otherwise liberty can do great harm. Difference between having certainty in a view and for the view to be certain. What about the objection that certain thoughts can be harmful to society? (atheism, etc. ) Mill"s rebuttal: the usefulness of an opinion is itself an opinion -> infinite regress. But accepting that, one would have no more reason to permit freedom of thought than one would have to ban it. Mill: generally better to have expression of ideas both true and false, untested, become dead dogma. We go to sleep at for true beliefs, [the] post as soon as there is no enemy in the field .