Why didn’t Quirrell just Accio the Philosopher’s Stone to him (PS17)? He probably couldn’t. Here’s why:
Magic requires many elements, but a prime one is familiarity. That is why most spells are line-of-sight. The wizard must see what it is in order to charm/curse it. This has been said many times. [An excellent example is when Quirrell and Snape were both casting spells on Harry in the Quidditch match (PS11). As Hermione pointed out later, it was imperative that a caster maintain eye contact.—ed.]
But then how could Harry use an Accio spell to fetch his Firebolt when it wasn’t within his sight? Well, two points. 1) Harry knows his Firebolt inside and out, he is very ‘in tune’ with it so to speak; and 2) he knows exactly where it is. Now I would suppose that, even if he didn’t know where it is, his familiarity with it would suffice (if he were powerful enough).
I suspect that such magics have three levels of power: Any wizard worth his salt can Accio a visible object, a more powerful wizard is needed to Accio a familiar and location-known object, but not visible, and only the most powerful can Accio an object on familiarity alone.
Now Quirrell did not know what the Philosopher’s Stone looked like (it’s been locked up for ages), and I think more than just *seeing it* would equal familiarity for an object that was not in line-of-site. And he does not *absolutely know* where it is. So he loses on all three accounts.
But there are a few possible snags to this theory, specifically with regards to the Accio spell. Mrs. Weasley uses it repeatedly to remove things from Fred and George’s clothing (GF6), things that she wasn’t sure what or where they were. How can this work?
- She saw them put something in their pockets (not likely).
- She obviously sees that “something” is in their pockets. The pockets themselves are visible, they’re bulging out or something. There has to be something there, and she knows it (more likely).
And frankly, we have to consider the distinct possibility that she was all too familiar with the sort of things Fred and George likely had in their pockets. This very well might not have been the first time she’d been through this scenario with the two of them.
But even if she didn’t know exactly what they had, she probably could see that they had something, and that would be enough. Familiarity in this case means simply knowing for certain that something is there.
For example, if a wizard saw a tablecloth over an obvious object underneath, that person could Accio it into his or her hand. Of course, this could be verrrrry dangerous. It could be a bowling ball with teeth or something.
It would be different if there were an object in a tablecloth in a room not in view. Then there’s nothing there as far as the person knows, hence no Accio.
So then what about the Aqualung Harry briefly considered Accio-ing from a nearby Muggle village for the second task (GF26)? The idea was shot down in short order, so we didn’t hear it planned out in detail, but it seems likely that Harry would have had to make a trek into that town ahead of time and figure out where such an unusual object might be located and make himself familiar with it. If he didn’t it would be impossible to have Accio-ed it. Familiarity is key.
I think this can apply to a Portkey and other spells as well. You can’t create a Portkey to somewhere you do not know, and you might not even be able to Apparate to a place you do not know. How did the Death Eaters Apparate to Voldemort in the graveyard? Voldemort sent up a “flare” with the Dark Mark tattoo, a sort of Death Eater homing beacon.