I think the content of the tape is a self-conversation of a patient, and the Doctor is Simon. During the conversation, the Doctor kept guiding other personalities. If you want to solve the troubles encountered by the ontology, you must wake up the personality Simon. This hints at why Simon uses the code name Doctor.
It was mentioned in the film that Gordon is calm and never loses his temper. This can be compared with other personality dialogues on the tape. It is learned that Gordon may be just one of the personality manifestations of the patient.
The mention of Gordon killing his own family in the film is an inducing background set up for this split personality theme, and the purpose is to explain Gordon's motives for committing the crime (arousing Simon).
From the perspective of split personality, it is logical to think that the picture of Mary in the film may be dressed up as one of the patients' personality.
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