“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.
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.”
Cited from: How to Read a Book, originally written by Mortimer Adler in 1940
and revised with Charles van Doren in 1972