I am facilitating a Sunday morning discussion class on C.S. Lewis's "Mere Christianity." In the process I came across a wonderful 60 minute audio introductory lecture to "Mere Christianity" by Boston College
philosophy professor Peter Kreeft.
Also, I was pleased to discover a helpful outline/summary of the book here.
One of my favorite chapters in Mere Christianity is Lewis's chapter on "Hope." Here he touches on the theme that lies at the center of his life: human desire and longing as a pointer to God and heaven. We experience it as "joy". This feeling of longing and desire is what the Germans call "sehnzucht," which Louis Marcos explains as:
a German word that means "longing" or "yearning." Lewis often used it to signify moments in his life when he felt an intense, overwhelming desire for an indefinable, numinous "something" that was just beyond his grasp. For Lewis, the reality of these moments of longing (he more often calls them simply, "joy") and the stubborn fact that he could find no object for them, either within himself or the natural world, proved to him that their source must be supernatural (a "proof" of the existence of God and heaven that is generally referred to as the "argument by desire"). - From Louis Markos, Course Guidebook to "The Life and Writings of C.S. Lewis," published by The Teaching Company.
Perhaps Lewis's premier discussion of longing and desire may be found
in his masterful sermon preached at Oxford University, "The Weight of Glory", which some regard as
the best sermon of the 20th century.
Boston College philosopher Peter Kreeft dissects the subject in his internet article, "The Argument from Desire." He offers a longer discussion in his contribution to G.K. Chesterton and C.S. Lewis: The Riddle of Joy, (a wonderful book for anyone interested in GKC or CSL). Check also his audio lecture here.
Finally, I will simply add that I thoroughly enjoyed and profited from a book I read shortly after it was published in 1974, Bright Shadow of Reality: C.S. Lewis and the Feeling Intellect by Corbin Scott Carnell. It is still in print, though with a revised subtitle: "Spiritual Longing in C.S. Lewis." Highly recommended.