Most discussions on immersion or ontology will involve someone using an example from sci-fi. Janet Murray's Hamlet on the Holodeck uses the imaging system from Star Trek to raise questions about what is real in a totally synthetic environment. A movie like eXistenZ, which you can find a brilliant essay on here, uses video games to discuss multiple layers of perceiving reality by having the characters play games within games. By the end of the movie, no one is really sure if reality itself has just become another layer of perceived existence.
While these talking points are useful, I've always been bothered by their reliance on the basic scenario of "Hey, what if I can totally simulate reality?" It's a great way to get the conversation rolling, but that' only one angle on something like ontology. I decided to do a write-up cataloging Philip K. Dick's ideas on reality that tackles several different approaches. If you're willing to acknowledge that a robot has feelings and is sentient, what does that say about you? What does that say about the very concept of emotion? Dick goes so far as to point out that you don't even actually need people to be plugged into some exotic machine for them to take games seriously, just the right cultural scenario.
Dick postulated that there was no really definitive reality. That it's all just perceptions on pure information. That in a very real sense, reality does not exist. I'm not sure I can ever go that far personally, but I think a good ontology discussion might benefit by asking yourself a more basic question.
Real compared to what?