Why is marking a book indispensable to reading it? First, it keeps you awake — not merely conscious, but wide awake. Second, reading, if it is active, is thinking, and thinking tends to express itself in words, spoken or written. The person who says he knows what he thinks but cannot express it usually does not know what he thinks. Third, writing your reactions down helps you to remember the thoughts of the author.

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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.”

How to Read a Book, scrisa de Mortimer Adler in 1940