From a 1930 letter from C.S. Lewis to his friend Arthur Greeves (from "Yours, Jack"):

"I envy you your shelf of MacDonalds and long to look over them with your guidance. I have read both The Princess and the Goblin and The Princess and Curdie. In fact I read the former (the other is a sequel to it) for about the third time when I was ill this spring. Read it at once if you have it, it is the better of the two. There is the fine part about the princess discovering her godmother in the attic spinning…. Another fine thing in The Pr. and the Goblin is where Curdie, in a dream, keeps on dreaming that he has waked up and then finding that he is still in bed. This means the same as the passage where Adam says to Lilith: ‘Unless you unclose your hand you will never die and therefore never wake. You may think you have died and even that you have risen again: but both will be a dream.’

This has a terrible meaning, specially for imaginative people. We read of spiritual efforts, and our imagination makes us believe that, because we enjoy the idea of doing them, we have done them. I am appalled to see how much of the change which I thought I had undergone lately was only imaginary. The real work seems still to be done. It is so fatally easy to confuse an aesthetic appreciation of the spiritual life with the life itself—to dream that you have waked, washed, and dressed, and then to find yourself still in bed…."