A patient describes an unwanted thought that keeps intruding into their mind. Before revealing the answer, decide what single question distinguishes an obsession from thought insertion.

Ask who the patient believes generated the thought in the first place. An obsession is recognised as the person's own thought, however unwelcome and however hard to dismiss, and the person typically retains insight that it is excessive or irrational. Thought insertion involves a conviction that the thought was placed into the mind by an outside agency and is foreign in origin, not simply unwanted. Both can be distressing and resisted, so distress and resistance do not distinguish them — ownership does.