I kind of get where you are coming from. But I really don't think that looking at the Outside through the IF (if that is what it does) necessarily suggests that Outside extends into the world
Why not? If the Inchoroi didn't believe that the Outside was connected to them in some personal way, they wouldn't be so afraid of it. By being able to create a way to see it, it is inherently connected to this space/time in some way.
and, in certain cases, when it does so it enables some individuals to be able to work sorcery (or whatever they might theoretically call it without being exposed to sorcery first).
I don't know if we're missing each other
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This is where the disconnect is, I think.
Curethan is just saying that when the Inchoroi discovered that there was something outside of their realm of physics/reality/science, they might have theorized ways that it (the outside) interacted with the known universe. Since it interacted (somehow) with them, there must be (theoretically) a way to interact with it. Without sorcery and the preceptory organs to grasp the Onta, they where never able to move beyond theory.
Once they discovered sorcery, they found could put their theories into practical use. All they needed was a little genetic grafting and a teacher...
This assumes, I guess, that the Outside is indeed responsible for Sorcery. If what the IF shows is not the Outside, or if the Gods/meta-physics are somehow not connected with the Outside the IF shows, or if none of it is connected to sorcery, and if the Inchoroi knew all that to be true, then Sorcery would be have been a complete surprise.
Even if the Inchoroi couldn't conceive of a way to use the Outside (i.e. Sorcery) to alter the world, they would at least have dismissed it as an impossible scientific construct. Once the saw Sorcery, it would have tipped them off that they were wrong. The Inchoroi might not even see a distinction between Sorcery and mundane Physics. Afterall, their Tekne was able to dupulicate the ability to wield it. They simply extended their scientific know-how into the the "spiritual" realm.
Hope that didn't just make things more confusing.