What does the initial allegation actually claim? To what part of the allegation is Mill objecting? What are the specific claims or premises Mills own argument relies upon? In your summary you should be as specific as possible and reference the reading directly. You should also aim to demonstrate your own understanding of the arguments (both the allegation and Mills response) by coming up with your own examples.
Because this is an argument reconstruction, you should not spend time explaining whether you agree or disagree with Mills argument. Focus instead on the reasoning he gives. We want to see the case he is making. We want to see the claims, or evidence or assumptions he is employing in support of his conclusion.
