Early in "The Signal-Man," the narrator suspects that the signalman might be suffering from mental illness, as he comments in the text:
"I have speculated since, whether there may have been infection in his mind."
In addition, he also wonders if the signalman might, in fact, be a ghost:
"The monstrous thought came into my mind, as I perused the fixed eyes and the saturnine face, that this was a spirit, not a man."
The narrator suspects these things because of the signalman's unusual demeanour. When he first arrives at the signal box, for example, he notices something "remarkable" in the signalman's manner which has no obvious explanation. Moreover, when the pair come face-to-face, the narrator is struck by the signalman's attitude of "expectation" and "watchfulness." Once again, the narrator is unable to account for such an attitude. Finally, when the signalman directs a "most curious look" towards the red light before looking at the narrator, the narrator's suspicions reach their peak: ghost or patient, the narrator cannot decipher his new friend.