St. Ignatius of Loyola (I think it was) advised his pupils to begin their meditations with what he called a compositio loci. [...] If I started with a compositio loci I should never reach the meditation. The picture would go on elaborating itself indefinitely and becoming every moment of less spiritual relevance.

There is indeed one mental image which does not lure me away into trivial elaborations. I mean the Crucifixion itself; not seen in terms of all the pictures and crucifixes, but as we must suppose it to have been in its raw, historical reality. But even this is of less spiritual value than one might expect. Compunction, compassion, gratitude - all the fruitful emotions - are strangled. Sheer physical horror leaves no room for these. Nightmare. Even so, the image ought to be periodically faced. But no one could live with it. It did not become a frequent motive of Christian art until the generations which had seen real crucifixions were all dead.
--C. S. Lewis, Letters to Malcolm: Chiefly on Prayer