The Mirror Protects the Stone

Quirrell cannot retrieve the Philosopher’s Stone; Harry sees his reflection pocket it and it appears on him. Dumbledore later explains that only someone wanting to find the Stone but not use it could obtain it.

Access control is keyed to latent intent rather than a stated request: the dangerous capability is released only when the user’s objective is compatible with non-use.

The mirror has magical, perfect mind-reading and no false positives; Dumbledore’s rule is explained only afterward.