In Resilience: Hard-Won Wisdom for Living a Better Life, Greitens, a former Navy SEAL and founder of the nonprofit the Mission Continues, offers a fellow SEAL advice from which anyone facing serious challenges can benefit.

What made you decide to become a SEAL?

Before I became a SEAL, I’d done humanitarian work around the world—with refugee families in Bosnia, with unaccompanied children in Rwanda, with kids who lost limbs to land mines in Cambodia. I’d seen incredible examples of resilient families facing great loss and pain, and I’d come to believe that, sometimes, the world needs strong people to serve and to lead. It isn’t enough to talk. Also, I wanted to be tested. Young men often seek tests and trials, and to me, BUD/S training—basic underwater demolition/SEAL—seemed like the ultimate test. Our class started with over 220 people. Only 21 graduated.

What is one lesson you learned in BUD/S that you’ve applied to your life in general?

What matters is what you do. And this runs counter to what a lot of the culture teaches people about putting feelings first. By contrast, resilient people focus not on what they intend, but on what they achieve. I’ve got a three-month-old at home now. And I can guarantee you that what matters is not whether or not you wanted to feed the baby, but whether or not you actually fed the baby.

Most of the book is made up of letters to another SEAL. What made you decide to share these?

Well, these are letters to my friend Zach Walker. Zach was a tough guy. Came home from Afghanistan greeted as a war hero. He started his own concrete business, was a good father to his two kids. Then he was hammered by hardship. His brother died. Lost his business. Drove home one day and stepped out of his truck and fell to the ground because he thought a sniper was watching him. Was diagnosed with a post-traumatic stress disability. Started drinking. Called me after he’d been arrested. We talked, and later that night I wrote him a letter. He wrote me back. I wrote to him again. Now, his story is unique but what he was up against—loss, fear, a search for purpose—is not. So, with Zach’s blessing, I collected these letters.

How is Zach doing now? What was his reaction to the book?

Zach is doing very well. He’s a great father to his three kids, coaching in the local community, and working on his farm. Zach and his family loved seeing the book in print, and they hope—as I do—that these letters will do as much good for other people who are struggling as they did for Zach.