/jp/ - Otaku Culture

In some sense? Sure.

I mean, which requirements does something need to fulfill in order to be "real"? From what we as situated perceivers in this world can tell, real things generally exist independently of us, can be coherently talked about and interacted with, and causally interact with other real things.

Now go and test Gensokyo against this list: Does it exist independently of us? Probably not, so that's a big minus. But it _can_ be coherently talked about and even interacted with to some extent. Given that there are hundreds of thousands of people out there who are acquainted with at least some of the source material, there's even some level of shared knowledge about Gensokyo. You can coherently talk about Gensokyo, make correct and wrong statements about Gensokyo and interact with it (albeit to a limited extent) via the games.

Finally, is there causal interaction between Gensokyo and things anyone would immediately consider real? That depends on your stance regarding causality and ontology. Reductionists would certainly deny this, but I do believe there's an argument to be made for considering Gensokyo and its constituents to be causally effective, especially when one employs a counterfactual theory of causation. You need to ask yourself: Would X have happened if Y had not been present? Would I have bought this Marisa fumo if Gensokyo had not been present?

Phenomenologically speaking, the borders of the world are wherever the borders of your direct or indirect perception are. But this also means that the world starts where your perception does.
If you believe Gensokyo to be real, it's already more real than something noone's thought about so far (or something that noone thinks about anymore). Add that to all of the above, and it's hard to think of Gensokyo as "mere fantasy" anymore. It's similar to how religions work - when your idea, your fantasy passes a certain threshold of requirements, it becomes more than just an idea. It can influence the real, material world, influence even people's lives, to the point that it doesn't matter anymore that this fantasy isn't material, isn't touchable. When you truly believe in a world after death, you don't question whether you will actually reach that world once you die, just as you don't question whether your computer is currently trying to deceive you by showing you a 4chan page when you're actually on reddit.

It is this deep, natural, everyday belief that makes almost everything we usually interact with real. We _have_ to believe that things we interact with will work in a certain way, that the world stays stable, and so on.
If you can get your belief about Gensokyo to that level... It would be hard to tell it apart from the real world at all.