"Most people, if they had really learned to look into their own hearts, would know that they do want, and want acutely, something that cannot be had in this world."

Edward M. Cook uses this passage as a springboard for a fascinating discussion of Lewis's concept of "joy" in his essay "Does Joy Lead to God? Lewis, Beversluis, and the Argument from Desire":

"Readers of Lewis will recognize in the 'desire' of Mere Christianity the experience called 'Joy' in Lewis's autobiographical Surprised by Joy:  The Shape of My Early Life (1955). In it, Lewis makes it clear that he knew all about the three ways of dealing with 'Joy' from personal experience:  he had tried them all.  From an early age, Lewis had repeatedly had experiences of intense longing for he knew not what; they were triggered sometimes by art, sometimes by poetry, sometimes by nature, sometimes by erotic experience.  So keen was the desire, and yet itself so desirable, that he returned again and again to what he thought were its sources; but he found that, although sometimes the 'Joy' would repeat itself, more often it would not, and with increasing rarity as he consciously sought it."