Higher Reality Therapy: Nine Pathways to Inner Peace - Page 160, Anthony Falikowski - 2010
A Level Psychology Through Diagrams - Page 274, Grahame Hill - 2001
OneLook Dictionary Search retrieved 28 October 2013 http://www.onelook.com/?w=intruder&ls=a
Maximum Potential an American Possibility - Page 2, Richard Monts 2010
Richard hanley, South Park and Philosophy: Bigger, Longer, and More Penetrating p 91, 2013
Bruce Hyman, 2009, The Ocd Workbook p 27