Adler begins with a simple distinction: owning a book physically is not the same as owning it intellectually.

You can have shelves full of unread books and still not be a serious reader. What matters is not what you own , but what you’ve absorbed . As Adler puts it, "Full ownership comes only when you have made [the book you’re reading] a part of... See more

This is why, in Adler’s view, marking a book doesn’t damage it. Rather, it leaves evidence — evidence that the reader has been there and grappled with the material. Just like a carpenter leaves dents on a well-used bench, so does a reader leaves ink on a well-read book.

But in 1940, a man named Mortimer Adler said this attitude was nonsense. His short essay entitled How to Mark a Book argued that writing in your books isn’t defacement — it’s a sign of life. The only way to really understand a book is to engage with it, argue with it, and respond to it.