Paul wrote a letter to Philemon regarding Philemon's runaway slave Onesimus, whom Paul had converted while in prison. In the letter, Paul refers to Onesimus as his "child" that he had "begotten" while in chains, and asks Philemon to accept Onesimus back not as a slave but as a brother. Paul pleads his case by reference to his role as a spiritual father and authority over Onesimus, noting the transformation in Onesimus from being useless to being useful, and promising to repay any debts Onesimus owed.