In its more extreme forms persecution mania is a recognised form of insanity. Some people imagine that others wish to kill them, or imprison them, or to do them some other grave injury. Often the wish to protect themselves against imaginary persecutors leads them into acts of violence which make it necessary to restrain their liberty. This, like many other forms of insanity, is only an exaggeration of a tendency not at all uncommon among people who count as normal. I do not propose to discuss the extreme forms, which are a matter for a psychiatrist. It is the milder forms that I wish to consider, because they are a very frequent cause of unhappiness, and because, not having gone so far as to produce definite insanity, they are still capable of being dealt with by the patient himself, provided he can be induced to diagnose his trouble rightly and to see that its origin lies within himself and not in the supposed hostility or unkindness of others.