Dr. Daniel Pierce is a brilliant lecturer at a Chicago university who often helps out the FBI on cases which concern the workings of the mind.
Pierce also suffers from hallucinations when he's not on his meds, but the people who appear to him invariably provide the sub-conscious clue needed to break the case.
Usually there's one major hallucination per case, which somehow ties into the investigation. And 'Possession' was supposedly no different with regular visits from the Devil.
I'm sure you noticed I said "supposedly" - because it's the contention of Toobworld Central that the Devil was not an hallucination but in fact, the Devil himself!
Pierce may have seen the Devil as a figment of his imagination, which is why he had no qualms about signing away his soul at the end of the episode. But I believe the Devil just wanted to make Pierce think that and readily played along with the scenario. In the end it would just make it easier to lure Pierce into signing away his soul.
The use of trickery is part of the Devil's game plan and he doesn't care about playing by the rules when it comes to garnering souls. He did the same thing to a man who so believed in UFOs that he gave away his immortal soul for the chance to have an actual encounter. Too late, that guy discovered he had been tricked and the spaceship full of the Greys about to welcome him was in fact a Hellmouth opening up to bring him into the "embrace" of the demons waiting to torment him for eternity.
There was an actual hallucination in the episode - that of the woman whom Pierce admired and romanticized. Natalie/Caroline was able to see the Devil as well, but that doesn't make him an hallucination along with her. She was merely acting out what Dr. Pierce's sub-conscious wanted - in this case, verification from another source that the Devil was real.
There's a reason why I'm so determined to make this theoretical connection work. John Glover - who played the Devil in this episode - also played the Devil on a weekly basis back in 1998 on the dark fantasy series 'Brimstone'.
The premise for that show had 113 souls escape from Hell and the Devil had the soul of a formerly corrupt cop seek them out and send them back to their eternal punishment.
What makes it even more important for this televisiologist is that there's also a theoretical link from 'Brimstone' to 'Buffy The Vampire Slayer':
When the vampire Angel/Angelus was dispatched back to Hell and off the show, it took some time before he was able to return on a regular basis. The reason given was that he had finally been able to escape from Hell.
A fellow Iddiot, known as Listener Mara, suggested that it meant Angel had been one of the 113 souls in that mass escape. We just never saw it happen in any episode, and with 'Brimstone' lasting only a season (if that!), we never would.
So if I make the claim that Glover was playing the actual Devil and not an hallucination, then I can claim that 'Perception' can be linked to 'Buffy The Vampire Slayer' and 'Angel'.
'Buffy The Vampire Slayer'
This episode is now in contention for the Toobits Award as the Best Theoretical Crossover of 2014......