I had the most magical experience a few weeks ago.

It wasn’t exactly time travel, but it felt like something close.

I was sitting down to work on a chapter for my next book (btw, ​the third book​ in the Stoic virtue series comes out in June. I’m working on the fourth now). I had decided to write a chapter on the importance of keeping what’s called a ​commonplace book​.

I sat down at my desk, pulled out my notecards, and found an old, worn notecard mentioning something that Joan Didion had written about notecards from a chapter in her book ​Slouching Towards Bethlehem​. I walked over to the shelf and pulled it down and of course, there it was, a beautiful essay in that book called “On Keeping a Notebook”, written in 1966.

I got goosebumps, not just because it was exactly what I needed, but because I happened to be sitting, at that very moment, in Joan Didion’s chair (​I bought it at a charity auction​ after her death). How did I know, nine years ago when I read ​Slouching Towards Bethlehem​, when I took the time to jot that little reference, that it might be of use to future-me?

“Why did I write it down?” Didion herself asks in that essay. “In order to remember, of course, but exactly what was it I wanted to remember?”

I don’t know, you never really do, but the process of finding, years later, the perfect thing that I had recorded in the margins of a book or a notebook, has happened to me so many times now that I’ve begun to question the time-space continuum.

When I was writing ​Courage is Calling​, for instance, I decided I would write about the Spartans at Thermopylae. I went to my shelf again and found there, in my ​Penguin Classics edition​ on page 477, what was effectively a highlighted outline of everything I needed to write this section…which had sat there silently for nearly twenty years. I didn’t even think I would be a writer when I read that book! I was just reading something that I thought was interesting!

This happens time and time again. One of my favorite books to re-read is F. Scott Fitzgerald’s ​The Great Gatsby​. I love ​Gatsby​ not just because it’s an incredible book, one of the great works of the English language. I love it because it was one of the first books I ever loved. I was assigned to read and write an essay on ​Gatsby​ in my sophomore English class and I still have that copy. So when I re-read ​Gatsby​, I’m not just talking to Nick Carroway and Jay Gatsby and Meyer Wolfsheim and Scott Fitzgerald himself, I am also talking to 16-year-old me. I can see the food I spilled while I read it at the kitchen table of my parent’s house. I can see my teenage handwriting in the margins.

I can also see the things I noted when I re-read it in college. I can see the notes I took when I read it in my twenties. I can see how I barely noticed the passages on page 73 the first few times I read it and I can see myself flipping back through the book to find them in 2016 when it suddenly hit me that the scene with Meyer Wolfsheim–a stand-in for the gangster Arnold Rothstein, fixer of the 1919 World Series–would be perfect for the opening of the book I was writing about Peter Thiel’s secret lawsuit, the book that would become ​Conspiracy​.

Even as I write this paragraph right now, I have ​Gatsby​ on my desk to revisit some of my favorite pages. I’m struck again by those first few sentences that I’ve read dozens of times: “In my younger and more vulnerable years my father gave me some advice that I’ve been turning over in my mind ever since. Wherever you feel like criticizing anyone, he told me, just remember that all people in this world haven’t had the advantages you’ve had.” Those words have different meaning to me today than they did five years ago, let alone when I first read them at 15. Now I have kids, now I have a better sense of my own advantages in life, now I know how hard it is to write something that good without sounding preachy or lame.

The poet Heraclitus talked about how we never step into the same river twice. By that he meant that the river is always changing, glowing evermore towards the sea, and we ourselves are changing, growing, getting older. The pages of a book don’t change, but we change, the world changes around them–we’re able to see and perceive things differently.

That’s one of the things that Didion notes in her essay on notebooks. Notebooks, she said, are a way “to keep on nodding terms with the people we used to be.” They are blasts from the past, reminders of how easily “we forget all too soon the things we thought we could never forget.”

I’ve talked before about my notecard system–which I learned from Robert Greene–so I won’t bore you with it here (​here’s a video about it​). But the reason I try to be an intentional reader, why I try to take notes and record and store what I read, is because I have seen the magic that comes from it, personally and professionally.

The best time to have started a notebook or a commonplace book would have been many years ago, but the second best time would be now. Start small–record what strikes you, quotes that motivate you, stories that inspire you. Don’t think too hard, just follow your curiosity. When you read a book, write in it, fold the pages, really engage with the material. Preserve this moment in time. Capture what you’re thinking and feeling. Your future self will thank you.

“It all comes back,” Didion writes at the close of her essay. More often than not, it will come back to you in ways that you couldn’t have planned for, but that you prepared for.

As a fellow time-traveler, I can tell you she’s right.

So start.