Reading a book should be a conversation between you and the author. Presumably he knows more about the subject than you do; if not, you probably should not be bothering with his book. But understanding is a two-way operation; the learner has to question himself and question the teacher, once he understands what the teacher is saying. Marking a book is literally an expression of your differences or your agreements with the author. It is the highest respect you can pay him.
Mortimer AdlerSin is not only manifested in certain acts that are forbidden by divine command. Sin also appears in attitudes and dispositions and feelings. Lust and hate are sins as well as adultery and murder. And, in the traditional Christian view, despair and chronic boredom - unaccompanied by any vicious act - are serious sins. They are expressions of man's separation from God, as the ultimate good, meaning, and end of human existence.
Mortimer AdlerEmancipation of human labor from economic servitude and exploitation, i.e., from organizations of production in which the conditions of work are determined by a master class who own the means of production, and in which the fruits of work are alienated from workers to the benefit of masters.
Mortimer Adler