Evidence suggests that Gideon's version of events may not be truthful or accurate. He is an unreliable narrator whose words should be viewed with some scepticism.
Gideon’s account of the magical appearance of the standing stone in Keldo Woods and his miraculous rescue by the Devil are fantastical. The reader has to suspend disbelief completely to accept them as real.
Gideon may lie about the nature of his affair with Elsie Moffat in order to protect her. He claims their affair was very short-lived and that she rejected him. However, Elsie reveals to Harry Caithness that their adultery lasted for years and that Gideon may be the father of her daughter:
both of the girls look like me so I’ll never be certain. But yes, I think Katie is Gideon’s"
The reader must decide whether to believe Gideon or Elsie, or to believe in a different truth that neither fully explains.
Almost everyone interviewed by Harry Caithness during his investigation believed that Gideon’s story was not true and that he had either gone insane or had suffered severe trauma after his accident.
Gideon’s mental state following his accident combined with the supernatural aspects and inconsistencies in his account invite the reader to question Gideon’s reliability as a narrator. On one hand his story could be completely accurate and truthful; on the other hand, his story could be the result of delusions caused by illness.
Gideon himself states:
this is my testimony. Read it and believe it, or believe it not. You may judge me a liar, a cheat, a madman, I do not care.
Robertson provides enough information to suggest that the events of the book could be real or imagined. But he is careful to not give so much evidence that one version of events seems more definite than the other.