You can put this another way by saying that while in other sciences the instruments you use are things external to yourself (things like microscopes … - C. S. Lewis

The instrument through which you see God is your whole self. And if a man's self is not kept clean and bright, his glimpse of God will be blurred

"I think you've seen Aslan," said Edmund.
"Aslan!" said Eustace. "I've heard that name mentioned several times since we joined the Dawn Treader. And I felt - I don't know what - I hated it. But I was hating everything then. And by the way, I'd like to apologise. I'm afraid I've been pretty beastly."
"That's all right," said Edmund. "Between ourselves, you haven't been as bad as I was on my first trip to Narnia. You were only an ass, but I was a traitor."
"Well, don't tell me about it, then," said Eustace. "But who is Aslan? Do you know him?"
"Well - he knows me," said Edmund. "He is the great Lion, the son of the Emperor-beyond-the-Sea, who saved me and saved Narnia. We've all seen him. Lucy sees him most often. And it may be Aslan's country we are sailing to."

There have been men before … who got so interested in proving the existence of God that they came to care nothing for God himself… as if the good Lord had nothing to do but to exist. There have been some who were so preoccupied with spreading Christianity that they never gave a thought to Christ.

It is funny how mortals always picture us as putting things into their minds: in reality our best work is done by keeping things out.