The narrator of Charles Dickens's story "The Signalman" wonders about the sanity of the man to whom he has called from above the railroad line and the tunnel.  For, when the narrator calls to him, the signalman turns back and forth in a strange way as though he is not certain from where the narrator's voice comes.  Also, after the narrator descends to speak with the signalman,  he notices that the railroad employee looks at the red warning light and then down the tunnel and then oddly at the narrator.  At this point, the narrator wonders,

...that this was a spirit, not a man.  I have speculated since, whether there may have been an infection in his mind.

Then, however, the narrator detects a hidden fear, a dread almost, in the signalman's eyes which "put the monstrous thought to flight."  After asking the signalman why he has reacted so, the man replies, "I was doubtful whether I had seen you before."  And, as they converse the narrator reveals that he comes to realize that the signalman has a sound mind and is a careful and conscientious worker.  But, the signalman will not disclose the reasons for his dread and consternation; yet, he promises to tell the narrator if he comes the next day.