Most spirits are unable to interact with the physical world. This restriction did not come about due to physical laws, but rather was implemented due to the actions of Puppet Master. This first spirit found that he could directly control the action of any human by sticking his hands into their brains (without even hurting them). He's also invisible, for everybody other than Spirit Spy, that is.

Strength

Can control humans.
Invisible.

Strength Source
Innate ability.