Sci-fi in pretty fantasy clothes?

[Swank]

Quite right. But the line between magic and telepathy is even thinner than that between science fiction and fantasy. The existence of telepathy has never been experimentally proven, nor has the existence of magic, and if telepathy or magic existed, people could use it to talk to each other mentally.
So in many cases the difference is just one of terminology. For example, there is the Darkover series of books by Marion Zimmer Bradley. It takes place on a lost colony planet where society and technology have been reduced to medieval levels, and some people have acquired abilities they call magic. As far as I remember, the reason for these abilities was their interbreeding with the native humanoids. If you take away the backstory about the lost colony and replace the native humanoids with some elves, you could easily turn the Darkover book series into a fantasy.

The line between magic and telepathy is that the author has picked one or the other and lets the reader know exactly that. Pern may fool the reader into believing it's a fantasy world at first, but that's a joke the McCaffrey is playing on the read. She eventually makes it quite clear that the "dragons" aren't mythical immortal monsters, but alien animals with clear biological mechanism that explain their appearance. And that's important because the author goes on to show how Pern isn't a fantasy, but an SF exploration of the possible in the real world - with all that implies. And that's world-building.

 

[Peppa]

Of course I understand that. But I was referring to situations where the imaginary world definitely cannot exist, but the author still tries to give a pseudo-scientific explanation for the existence of strange creatures and magic in this world.

 

[Swank]

Of course I understand that. But I was referring to situations where the imaginary world definitely cannot exist, but the author still tries to give a pseudo-scientific explanation for the existence of strange creatures and magic in this world.

I categorically believe that the difference between fantasy and SF doesn't come down to a one paragraph explanation about where some power comes from. It is fundamental to the entire story and can be seen throughout.