La Miroir Vivant par Magritte, 1927
From The Problem of Pain
  by C.S. Lewis (1898 - 1963) and
published in 1940.
THE HUMAN spirit will not even begin to try to surrender self-will as long as all seems to be well with it. Now error and sin both have this property, that the deeper they are the less their victim suspects their existence; they are masked evil. Pain is unmasked, unmistakable evil, every man knows that something is wrong when he is being hurt.... And pain is not only immediately recognizable evil, but evil impossible to ignore. We can rest contentedly in our sins and in our stupidities; and anyone who has watched gluttons shoveling down the most exquisite foods as if they did not know what they were eating, will admit that we can ignore even pleasure. But pain insists upon being attended to. God whispers to us in our pleasures, speaks in our conscience, but shouts in our pains: it is His megaphone to rouse a deaf world....
This paragraph is from Chapter 6 of The Problem of Pain, and the picture is The Living Mirror by Magritte.