In Mere Christianity, C.S. Lewis described faith as "the art of holding on to things your reason has once accepted, in spite of your changing moods." His is a crucial observation for a world that often pits reason against faith. Lewis understood that faith must always be guarded against the assaults of changing emotion.

Lewis powerfully illustrated this point in The Silver Chair, the fourth book of The Chronicles of Narnia series. The story opens with Jill Pole, a typical English schoolgirl, being called suddenly (and even more strangely than anyone before her) into Narnia. Aslan, the Great Lion, gives her the task of rescuing Prince Rilian, son of Caspian, who had been missing for ten years. To help her, Aslan gives Jill signs to recite and remember, along with this dire warning: 

"Here on the mountain, the air is clear … as you drop down into Narnia, the air will thicken. Take great care that it does not confuse your mind."

 

Jill learns quickly just how true his warning is. Eventually, having left the surface of Narnia and descended to the depths of the underworld, she, Eustace Scrubb, and Puddleglum the Marshwiggle find Narnia's lost prince. He's so deeply enchanted by the Witch's dark magic that he can no longer tell madness from reality, truth from lies. It's only in the full grasp of his "madness," which actually turns out to be his moments of lucidity, that the prince unknowingly invokes the final sign given to Jill: he calls on the name of Aslan. 

In that moment, Lewis masterfully portrays the fog of doubt and deception. Under the Witch's enchantment, it's not clear who is a friend and who is an enemy. In fact, the three adventurers feel sure that the prince will attack them the moment he is free, but as Puddleglum reminds them in a moment of powerful courage, they've sworn to obey the words of Aslan. Only that better commitment, which might be called the right ordering of their loves, sees them through. They cut Rilian loose and break the Silver Chair, destroying the power of the Witch's curse.

Lewis, of course, knew what it was to struggle with doubt. "Now that I am a Christian," he wrote, 'I do have moods in which the whole thing looks very improbable: but when I was an atheist, I had moods in which Christianity looked terribly probable." That's why faith mattered to Lewis: it grounds us in reality, even in the face of danger or uncertainty. 

Today, a generation of young people are debilitated by feelings of meaninglessness, doubt, and depression. They consistently hear that their feelings are their best and highest guide; they're encouraged to look inside and follow their hearts. Aslan's advice is better: "Remember the signs." In other words, only by looking to fixed, sure reference points outside of ourselves, can we orient and know the way forward.

When the Witch returns to the cave, attempting to deceive Jill, Eustace, and Puddleglum again, she offers us a dialogue that could substitute for modern textbooks on epistemology. 

"What is the sun?" the Witch asks the children, who have been underground for so long, all they have is a vague memory of things like Aslan, the sun, and the overworld. "It's like a lamp," one offers. But the Witch laughs this off. "Your sun is a dream, and there is nothing in that dream that was not copied from the lamp." In other words, "the lamp is the real thing; the sun is [just] a children's story." 

Materialism offers the same argument. Because the idea of God helped us survive, goes the argument, people came to believe in him as real. But all we're doing is taking concrete things around us and inventing fairy stories about their origins. Just as the sun can be forgotten in a subterranean kingdom, Christians can sometimes feel as if there is no immediate proof of God's existence. 

GK Chesterton addressed this default appeal to materialism. "As an explanation of the world, [it] has a sort of insane simplicity… we have at once the sense of it covering everything and the sense of it leaving everything out."  Materialism's explanation for love, goodness, evil, and personhood is comprehensive, but ultimately guts these things of any real substance. 

Likewise, in The Silver Chair, the sun, Narnia, and Aslan are real: in fact, they're the most real things of all. It's the Witch's kingdom that is the shallow copy. In the end, only Puddleglum the Marshwiggle can hold on to the truth, which leads him to stomp out the fire and break the Witch's spell for good. 

The solution to doubt is, then, according to Lewis, faith. Not blind belief, but a commitment informed by reason, goodness, and imagination. What God has told us in the light of day and which we then know to be true, we should not doubt in the middle of our darkest night. 

The only way forward is to, in the words of Aslan, "Remember the signs!"