In magick, a witness is something that stands for a person. Blood from the person, his/her hair, clothing, fingernails etc. can be used. Often a drawing (even a stick figure) or photo will do (an you though some cultures were just being superstitious).
The witness can be use to diagnose (dia-gnosis), heal, effect and manipulate if the person allows or has opened him/herself up to it.
The mechanics of the aura only allows things in that have passed out.