Excerpted from Mortimer Adler's How to Read a Book

But, you may ask, why is writing necessary? Well, the physical act of writing, with your own hand, brings words and sentences more sharply before your mind and preserves them better in your memory. To set down your reaction to important words and sentences you have read, and the questions they have raised in your mind, is to preserve those reactions and sharpen those questions.

Even if you wrote on a scratch pad, and threw the paper away when you had finished writing, your grasp of the book [or poem or essay or short story or play] would be surer. But you don't have to throw the paper away. The margins (top and bottom, as well as side), the end--papers, the very space between the lines, are all available. They aren't sacred. And best of all, your marks and notes become an integral part of the book and stay there forever. You can pick up the book the following week or year, and these are all your points of agreement, disagreement, doubt, and inquiry. It's like resuming an interrupted conversation with the advantage of being able to pick up where you left off.

And that is exactly what reading a book should be: a conversation between you and the author. Presumably he knows more about the subject than you do; naturally, you'll have the proper humility as you approach him. But don't let anybody tell you that a reader is supposed to be solely on the receiving end. Understanding is a two-way operation; learning doesn't consist in being an empty receptacle. The learner has to question himself and question the teacher. He even has to argue with the teacher, once he understands what the teacher is saying. And marking a book is literally an expression of your differences, or agreements of opinion, with the author.

There are all kinds of devices for marking a book intelligently and fruitfully. Here's the way I do it:

1. Underlining: Of major points, of important or forceful statements.

2. Vertical lines at the margin: To emphasize a statement already underlined.

3. Star, asterisk, or other doo-dad at the margin: To be used sparingly, to emphasize the ten or twenty most important statements in the book. (You may want to fold the bottom corner of each page on which you use such marks. It won't hurt the sturdy paper on which most modern books are printed, and you will be able to take the book of the shelf at any time and, by opening it at the folded-corner page [now is the time to use post-it notes instead], refresh your recollection of the book.)

4. Numbers in the margin: To indicate the sequence of points the author makes in developing a single argument.

5. Numbers of other pages in the margin: To indicate where else in the book the author made points relevant to the point marked; to tie up the ideas in a book, which, though they may be separated by many pages, belong together.

6. Circling of key words or phrases.

7. Writing in the margin, or at the top or bottom of the page, for the sake of: Recording questions (and perhaps answers) which a passage raised in your mind; reducing a complicated discussion to a simple statement; recording the sequence of major points right through the book. I use the end-papers at the back of the book to make a personal index of the author's points in the order of their appearance.

8. The front end-papers are, to me, the most important. Some people reserve them for a fancy bookplate. I reserve them for fancy thinking. After I have finished reading a book and making my personal index on the back end-papers, I turn to the front and try to outline the book, not page by page, or point by point (I've already done that at the back), but as an integrated structure, with a basic unity and an order of parts. The is outline is, to me, the measure of my understanding of the work.

9. If you're a die-hard and anti-book-marker, you may object that the margins, the space between the lines, and the end-papers don't give you room enough. All right. How about using a scratch pad slightly smaller than a page-size of the book--so that the edges of the sheets won't protrude? Make your index, outlines, and even your notes on the pad, and then insert these sheets permanent inside the front and back covers of the book.

10. Or, you may say that this business of marking books is going to slow down your reading. It probably will. That one of the reasons for doing it. Most of us have been taken in by the notion that speed of reading is a measure of our intelligence. There is no such thing as the right speed for intelligent reading. Some things should be read slowly and even laboriously. The sign of intelligence in reading is the ability to read different things differently according to their work. In the case of good books, the point is not to see how many of them you can get through you---how many you can make your own. A few friends are better than a thousand acquaintances. If this be your aim, as it should be, you will not be impatient if it takes more time and effort to read a great book than it does a newspaper.

Your may have one final objection to marking books. You can't lend them to your friends because nobody else can read them [your husband or wife may need his or her own copy or you may need to work out a plan for sharing without leaving marks for the other person's engagement with the book] without being distracted by your notes. Furthermore, you won't want to lend them because a marked copy is a kind of intellectual diary, and lending it is almost like giving your mind away.

If your friend wished to you're your Plutarch's Lives, "Shakespeare," or The Federalist Papers, tell him gently but firmly, to buy a copy. You will lend him your car or your coat--but your books are as much a part of you as your head or your heart.