The root text is Sahih Muslim 2814a: there is none among you but has a companion appointed over him from among the jinn, and the companions asked, even you, Messenger of Allah, and he said, even me, but Allah aided me against him and he submitted, so he commands me only to good. Ibn Taymiyyah systematized the mechanics in the Majmu' al-Fatawa: jinn and devils embody physical shapes humans can see, appear as the dead to the grieving, sit in the house while showing a man himself outside, and lend powers that recipients mistake for their own, and his al-Furqan bayna Awliya' al-Rahman wa Awliya' al-Shaytan is the tradition's field manual for distinguishing the saint from the impersonator, written in the fourteenth century against exactly the claims of borrowed faces and staged wonders.
Folk usage gives the qareen the night shift: the intrusive thought, the earworm, the sudden certainty at three in the morning that the room is occupied, all filed to the companion's account, the waswas, the whispering, being its known instrument. The double motif is universal and the tradition knows it, the qareen sits in a family with Egypt's ka, the Norse fylgja, and the doppelgänger whose sighting forecasts the seer's death, every culture staffing the same vacancy, a second you that arrived first and will leave last.
The claim: the qareen is the archive's cleanest model of the haunted mind, the inner voice defined as an external process with permanent access, and every later double renegotiates the same contract, the guardian angel, the higher self, the spirit guide, the channeled spokesman with a name like Tom, each one the companion promoted, baptized, or put on salary. Verdict: the texts are documented and the mapping is the archive's, filed as a reading, not a finding. The doctrine itself files the companion as hostile until converted, which is either a warning about the unseen or the oldest surviving description of an inner critic with administrator rights.
If the voice that knows you best was assigned at birth and never leaves, which of your thoughts were ever yours?