It's a demonic possession/influence.
I don't trust anyone who is 100% cocksure about something that is ambiguous...
The Director did not set the movie up to 'trick' people.
How did Gordon pick Mary's room, out of all the rooms...with his own family's pictures on the wall? Why did the moment Mike found the folder with Mary's grave number on it, it flashed to Gordon showing him sitting right next to it - plot 444.
The connection between Mary and Gordon, is Simon, which is odd, wouldn't you agree - because Gordon hasn't been exposed to Mary's tape sessions. He knows nothing about Mary, yet he's gravitating towards her room and her cemetery plot, for what reason? Just a coincidence? In real life, sure, but the Director focused on these items on for a reason. Was it just to confuse us poor little 'demon' based theorists? NO. Directors show specific scenes for specific purposes. It is quite a 'coincidence' out of a heavily populated cemetery that the guy that is weak and vulnerable based on his personal circumstances suddenly chooses to hang around plot #444. And no, the same voice that speaks for Simon is the same voice that Gordon hears...does not make sense in the 'purely psychological' theory. Also "Why did Mary let you? Simon: "They always do, doc, they always do' while laughing." Oh, so here's a completely different voice, a completely individualized entity referring to more than one person when asked about ONE specific person, "Mary". Why did MARY let you and the entity responds "THEY always do".
Notice when the doctor asks 'Simon' where do you live - the camera shots take on a completely new view - floating above the building.
This is about mental illness as a vulnerability and an opening to influences, in this case demonic.
Not very original, but 'We accept the love we think we deserve.' Brilliant.