Ghosts
Incorporeality
The majority of ghosts are unable to interact with the living world in a normal manner. They are incorporeal — they cannot touch physical objects, or move them. Attempts to do so usually results in them passing right through the object that they are attempting to interact with, be it a pencil, a table, or even a living being.
To produce even the most minimal of physical interactions with the living world, a ghost must know the proper procedure, and even then the results are extremely sketchy. The only one hundred percent assured way to contact the living is via a necromancer.1