"Remember, remember, remember the signs.  Say them to yourself when you wake in the morning and when you lie down at night, and when you wake in the middle of the night.  And whatever strange things may happen to you, let nothing turn your mind from following the signs.  And secondly, I give you a warning.  Here on the mountain I have spoken to you clearly: I will not often do so down in Narnia.  Here on the mountain, the air is clear and your mind is clear; as you drop down into Narnia, the air will thicken.  Take great care that it does not confuse your mind.  And the signs which you have learned here will not look at all as you expect them to look, when you meet them there.  That is why it is so important to know them by heart and pay no attention to appearances.  Remember the signs and believe the signs.  Nothing else matters.  And now, daughter of Eve, farewell--"

The Silver Chair, by C.S.Lewis (New York, 1953), p. 27