An illustration showing how we are taught that unclean spirits know the thoughts of men.
For just as some thieves are in the habit of examining the concealed treasures of the men in those houses which they mean to rob, and in the dark shades of night sprinkle with careful hands little grains of sand and discover the hidden treasures which they cannot see by the tinkling sound with which they answer to the fall of the sand, and so arrive at certain knowledge of each thing and metal, which betrays itself in a way by the voice elicited from it; so these too, in order to explore the treasures of our heart, scatter over us the sand of certain evil suggestions, and when they see some bodily affection arise corresponding to their character, they recognize as if by a sort of tinkling sound proceeding from the inmost recesses, what it is that is stored up in the secret chamber of the inner man.
Source: Conferences (New Advent)