An obsession is an intrusive thought or impulse that enters the mind despite efforts to exclude them. Most of the time, patients consider them false and totally against their own beliefs. For example, someone might think their hands are dirty and thus repeatedly wash their hands. But notice they clean their hands to reduce the distress of their intrusive thoughts. That is, they know their hands are clean, but due to the intrusive thoughts of dirt, they want to ensure that their hands are clean again.