It's interesting to me how some get strangely angry at any mention of the supernatural. Honestly, the OP is "shocked and appalled" that someone might have a different interpretation of a film which works pretty hard to allow for multiple interpretations. There's a lot of films with ambiguity to them yet I'm not "appalled" when someone has a different interpretation of the top at the end of Inception than I do.
I wonder if people who seem to have such an extreme reaction are perhaps bringing in their own personal vendettas against the supernatural into their viewing of the film. I'm only speculating but I've often seen die-hard skeptics to be as radical as fundamentalist Christians in their unwillingness to listen to other ideas.
Thank you! So well put! You know I personally don't care if they chose to believe the issue in the movie is more organic, more of a matter of Gordon's mental instability. Yes, I can see that as a point of view and I think it's one of many facets that makes this movie great. There are different ways of looking at it and it only makes the movie more entertaining to me.
I just don't like it when these people close out all possibilities. I for one believe there to be a supernatural cause. And I also agree with you about the personal vendetta argument. I have a friend who is almost religiously atheist. It's bizarre at how devout she is about it. She went as far to argue that The Exorcist had no supernatural theme whatsoever, it was all mental instability. This of course is nonsense.
Anyway, I digress, but I've noticed this same pattern in these posts. It can't be supernatural related and how dare you even suggest such a thing! Such a shame to be so closed minded.
American Horror Story Season 6: Donald Trump

It's interesting to me how some get strangely angry at any mention of the supernatural. Honestly, the OP is "shocked and appalled" that someone might have a different interpretation of a film which works pretty hard to allow for multiple interpretations. There's a lot of films with ambiguity to them yet I'm not "appalled" when someone has a different interpretation of the top at the end of Inception than I do.
I wonder if people who seem to have such an extreme reaction are perhaps bringing in their own personal vendettas against the supernatural into their viewing of the film. I'm only speculating but I've often seen die-hard skeptics to be as radical as fundamentalist Christians in their unwillingness to listen to other ideas.
I'm not saying I believe that one has to believe there's a supernatural element to this film. I'm only saying that it's obvious the filmmakers intended there to be more than one way for the film to be taken. The only "wrong" interpretation is the belief that there's only one interpretation for the movie.
There's a well known novella called
The Turn of the Screw
(and a great film adaptation retitled
The Innocents
) which had a similar controversy surrounding it. Without spoiling too much, half of the readers of the story leave with the impression that it's a story about a haunting and the other half believe it's a story about protagonist going slowly insane. Most modern critics have since become content that the story wants us to consider both options and decide for ourselves. That's what makes that story interesting and that's what makes Session 9 interesting.
I personally wouldn't have been so captivated by a film had it been a movie about a former mental patient who goes crazy again and murders his family. I also would not have been so captivated by the film if it had been just a story of a haunted hospital. The movie is interesting because it walks the line between these interpretations and that uncertainty gives it a level of real terror.

Nah, I'll go on believing what I want to believe regardless of what you say. It was supernatural. Simon even tells us at the end that this is the case and this isn't the first time he has stepped in when he says, "They always dothey always do." Of course he's referring to his victims letting him take control.
I gotta go feed that thing in Room 33.

There is a logical an rational reason for the electricity going off. It's an old building with an old wiring scheme. Old buildings + old panels+ large electrical equipment= electricity going out. I own an old bank building, our panel is old, when I have too many things plugged in, the same thing happens. The outdated panel can't handle the current load. I'm kind of shocked (pun not intended, well maybe a little) nobody brought that up yet.

What I really like about
Session 9
is, aside from the creepy score and great acting, it's mysteriousness and ambiguity, so I don't see the need for one right theory.
However, I believe the supernatural element does exist in the story and much of the "evidence" written above for this are excellent. I'd like to point out on a less serious note that the supernatural theories are presented more politely
I have seen the supernatural, specifically demonic, and psychological elements intermixed in a movie before, for example
The Excorcism of Emily Rose
.
The two elements can coincide because

1. possession and mental illness can sometimes be confused for the other,
2. the vulnerable and exposed state mental illness can possibly lead to higher risk of possession
   In
   Session 9
   , Mary has three alters and one of them only presents himself to the doctor in
   Session 9
   of Mary's meetings with him, and I agree with excellent supernatural theorists above that Simon is possibly a separate and evil entity (demon) that causes Mary, and then Gordon to murder people.

Why does this atheist keep blathering on about this movie being a psychological thriller with no paranormal story line? Insecurity problems? People voted and you lost, so get over it.

As far as the plot of the movie, I agree with you. It is clear to me that this movie is not supernatural. Your assessment makes sense that Gordon had to be a former patient in the asylum due to the deja vu moments, the pictures of his family, and the others. I agree with you assessment of the plot, but not on your assessment of DID.
I would encourage you to read this:
Sinason, V. (2012).
Trauma, dissociation and multiplicity: Working on identity and selves.
New York, NY, US: Routledge/Taylor & Francis Group
and if you don't have access to it in a library:
http://psychcentral.com/lib/dispelling-myths-about-dissociative-identity-disorder/0009785/2
First, one of the main reasons it was changed from multiple personality disorder to dissociative identity disorder is that the personalities are not distinct and separate personalities as Hollywood would like you to believe. They are generally distinct emotional states of the same personality. The states generally have the entire complexity of a certain emotion. For example, one state may be the only one that expresses any anger. Because the subject was unable to deal with things angrily, those emotions were diverted into a different state. Technically all of the "personalities" are simply different states of the same person. It is a therapist's job to integrate those emotional states into one complete being.
Gordon does not remember his agressive actions - also something DID patients do. The ''wake up'' part.
This is Hollywood hogwash and has no bearing in documented cases of DID.
Furthermore this movie has "Simon" or Gordon's "Simon" telling Mary and himself to commit violent crimes. A DID patient will never suddenly gain a memory that was hidden by a different personality or state. If the personality or state was unaware of the memory when it happened, they would remain unaware, generally even if the identities are integrated. But even if they do, the identities have to be integrated through therapy and it would not spontaneously happen as it seems to have happened with Gordon. This is completely unrelated to DID and someone showing such symptoms would be more likely diagnosed with schizophrenia. The whole point of dissociative identities seems to be to create different states of personality that are capable of dealing with emotions that the other personalities are incapable of dealing with. If Simon is indeed a dissociative identity, then he would be the identity that is able to deal with the violent nature the other identities are unable to deal with. That alone would preclude Simon from telling Mary to hurt anyone, because he would not exist if she were capable of doing so. Simon would be doing the violent acts himself without any input from the other identities.
Therefore, this may not be supernatural, but it is anything but a textbook case of DID.

trolling because of all the shots and all the moments when it would make you think it was a ghost movie.
Moments where it made it seemed like there was a paranormal explanation behind some things. But what made me say "Trolling" Was the emphasis he gives of the opening of the box and the reactions of everyone in the place.
He chose to make the opening of the box a big deal and ever show how everyone reacted like something was wrong even if they were not in the same room or if they knew what was going on.
As well as the whispers and things like that. He definitely made at least a couple of shots to feel like there was something else going on

The only thing that this post And this movie shows me is that the director of the movie is a HUGE troll.
the director is a troll, and anyone that got confused by the movie got trolled.
I disagree. I can see that logic for someone who sees two stories, but I probably watched
Session 9
five times before I even heard about IMDB. I never once considered this movie to have anything supernatural - I completely (and still do) consider it a movie about mental collapse. The supernatural view was first seen by me here.
So, to your point, the director is only a troll if
you
think he was trolling. I think he made a great, straight-ahead psychological horror. Art is interprative, and I'm not sure "trolling" existed in 2001, but I understand your point.
I'll always go back to this main point, though.
A movie about mental collapse amidst stress in an ominous setting: That's cool and unique.
A movie about getting possessed by an unseen force in an ominous setting: That's been done a hundred times before this.

This message has been deleted.

I agree with your comment: why have the electricity go off?
Point well taken, and I like that.
People get passionate about this film because it never got the recognition it deserved, and it's a very creative concept. The debate here is valid, and it's a great concept that's built into the writing and direction. Some films deserve more respect, and then there's some films that get too much attention.

i didn't want to call the director a troll but if I ever saw a trolling movie this is the one. Specially because of how angry and extremist people got in here,

Exactly tcm. Well stated. Thank you.

The only thing that this post And this movie shows me is that the director of the movie is a HUGE troll.
This guy trolled everyone that ever watched this movie. If this movie is only about mental sickness then WHY on EARTH add so many creepy moments or moments that felt supernatural?
Why have Simon at the end be a different voice?
Why put SO much Emphasis on the opening of the box?
Why have the creepy whispers that some of the characters heard around?
Why have the electricity go off?
All those things were added to troll the hell of the audience. Is that simple.
He was making a movie about D.I.D but decided to mess with people watching it so he added all these other horror things just to confuse everyone.
the director is a troll, and anyone that got confused by the movie got trolled.

How does it feel to always needing to be right? It must be exhausting.

That this conversation is still going strong 13 years after the movie's release is a testament to how well it was presented, how perfectly vague (or direct) it was, and how a movie's merit shouldn't be based on how shiny it is, how well acted by David Caruso, or by the box-office but how well
it sticks with you.
Carry on!

I think Simon was some kind of supernatural presence. For a few reasons: the generator always going out, the coins leading conveniently to the crematorium, the fact that Mike just so happens to find Mary's files in a gigantic hospital like that(having worked in several hospitals, I can say it's extremely difficult to find someones files on PURPOSE), also gordon just so happening to go bonkers as soon as they go to this hospital that also just so happens to have housed someone that went bonkers in a similar manner, gordon sitting right over mary's grave (out of like 700+ graves i think), at the end when asked where it resides simon says in the weak and wounded, also what are the odds that gordon coudl perform a lobotomy like that? I know mike gave a very crude explanation, but I find it kind of hard to believe that gordon could successfully perform one after hearing a seconds long description of the procedure. But that's not really here nor there, I think it's just probably something I need to suspend my disbelief for. Anyway, I find all these reasons to be compelling indications as to some type of supernatural involvement. Either that or they're just examples of poor/contrived writing. I think, however, the director has left it ambiguous on purpose and to say without a doubt that it is, in any way other than your opinion, one way or another would be presumptuous.
PS: About DID; I find a DID explanation almost as unrealistic as a supernatural one. I don't believe in the supernatural in real life, by the way. DID is extremely rare, so rare that whether or not it actually exists as it is popularly portrayed is a contentious subject. As far as I can tell, there has never been a DID-murderer or serial murderer. And how is Gordon's experience in keeping with DID etiology? Current literature speculates that it is most commonly found in young adults after an extremely stressful/traumatic experience. Gordon wasn't a young adult and the only thing traumatic he had experienced was the new addition to his family. Now I'm not trying to trivialize parenthood but I don't think that's a traumatic/stressful enough event to cause a heretofore stable man to turn into a cold, calculating psychopath. I also don't think Gordon was a former patient of the hospital. That would be an interesting twist, but I just don't see enough evidence to support it. Henry says that Gordon had been in the abestos business since before he came over here (to the US I think) and he does seem to have migrated from overseas because he had an accent. That's speculation, of course, because Henry might have been lying or Gordon might have been lying to him to cover up having stayed at the hospital.
So in conclusion I think you could convincingly argue for either side (supernatural or mental disorder), I think the supernatural explanation is a more fulfilling explanation. Maybe because I've seen too many DID scary movies here recently (Hide and seek, pandorum, identity, secret window, etc)and it frustrates me that writers just latch onto this extremely rare disorder and then blow its signs/symptoms way out of proportion .

I don't think it is a "ghost" in the traditional sense, but the film implies we are all able of the worst, that we are somehow haunted by something horrible that lies far in our inner worlds.
Gordon doesn't have multiple personalities, but Mary's story reveals something about him anyway, how he refuses to see what he has done. Simon could be his inner voice, like it is for the terrible things Mary has done.

I'm not at all discounting your theory as a possibility. But do you realize that the Wikipedia summary of this movie says that Simon encourages Gordon to murder his wife and daughter? This movie is from 2001. If Wikipedia's summary was a true error that was provable by fact and correctable, wouldn't someone have requested it be changed by now? When I read the Wikipedia, I thought, well, I'm not certain it actually was Mary's same Simon who influenced Gordon, but it's certainly a good possibility. After all, the film is called Session 9, and Simon is introduced in Session 9. Why would the filmmakers choose that title if Mary's story wasn't clearly intertwined with the film's present events (and not just as an example of D.I.D.)? I think there is too much evidence that Gordon was influenced by Mary's Simon (as some of the other posters above have pointed out about the grave, etc.) to completely discount that theory. After all, Gordon wasn't listening to Mary's tapes. Mike was. Gordon didn't know anything about her. Why would he be drawn to her grave if there wasn't something supernatural pulling him there? I agree with Zando777, who said, the movie is about "the ambiguity and horror of mental illness, and how it parallels traditional tales of demonic possession. To the point where it's essentially impossible to distinguish between the two."

If there was no supernatural element at play, then why was Gordon seemingly so attracted to the locales where Mary had been, such as her grave marker (above which he deliberately sat down), and later, why did he appear to move into her old room in the hospital? I read this as him being driven by the same entity which had been inside Mary, repeating its own history again and again within the hospital itself.
I like your theory, and I agree that Gordon and Mary both suffered from DID, but I do believe there was something else that Mary had brought with her from her home setting, to the hospital, and which had stayed there after her death, waiting for "the right kind of person" to come along.

While watching the movie I too thought one of them must have been a previous patient because of the way they brought up some of the patients sometimes returned but I believe the sub plot about the woman who used to be a patient was the reason for this. When this was taken out the viewer in trying to find a reason takes this as being the case especially because a character also says who would want to live here I would rather live on the streets and Gordon decides to live there when he has 'trouble' with his wife.

Yeah, I agree!

There is nothing in this movie whatsoever that suggests that Gordon was ever a mental patient anywhere, let alone at this hospital. This theory is what some have come up with to try and explain his sudden madness, but I don't think we can legitimately project that as its not in the movie at all.
Hydrotherapy was not a treatment that was ever used during Gordon's lifetime. It was used in the 19th century and the first half of the 20th century but discontinued once medications came on the scene, so its not something Gordon would have first hand knowledge.
Gordon was not in to Bill's reminiscing about the hospital's history. He was on the edge with his business which would have folded if he failed to get the asbestos contract to clean up Danvers. He wanted to get through the survey so he could make the deal with Bill; Gordon cut Bill off several times when he went off in to a tangent about the hospital. What was on Gordon's mind was the deal of his life since if didn't get that deal, his life would have crumbled around him. As it turned out, his life crumbled anyway because he made the deal.
The million dollar question is, why did it? Well, his life was on the edge because he was one deal of way from loosing his business and his livelihood. Also, he was sleep deprived because of daughter's earaches and her crying keeping him up at night for a very long time. These two things are reason enough for someone to crack and it happens all the time. Throw in the very bad vibe of the hospital, then, you have what happened.

First of all I have to say this is one of my favorite movies, and I struggle to understand why people dislike this film. When it comes to all these theories I agree with OP about the medical condition being the basis for this movie. However, I never thought about Gordon being a former patient.
When watching the film with this in my back head, I noticed Gordon really seemed as is he had been there before. Firstly, his reaction when Bill explains hydra therapy tells me he could have some experience with it. And some minutes later, Bill tells Gordon to hold up, after Gordon just startet leading the way. This could be signs of him knowing the place. But then again, how could Phil not be aware of this after knowing Gordon a long time? If Gordon was thrown out in the 80's, it would have been over 10 years, so I guess its possible.