I like this description:
The eye is the instrument by which we see everything, and for that reason it is the one thing we cannot see with truth. The same thing is true of our Power of response to a book, or to anything else; incidentally, this is why books about the Holy Ghost are apt to be curiously difficult and unsatisfactory-we cannot really look at the movement of the Spirit, just because It is the Power by which we do the looking.
-Dorothy Sayers, The Mind of the Maker, Ch.8
My wife commented that this is the same as Lewis’s frustration with trying to see and understand Joy. It turns out that Joy was the vehicle to see God, not the end in itself. It made it very difficult to focus on. You could only see the trail where it had been.