|Charms of Defence and Deterrence|
In this book, Professor Spangle set forth certain principles on the Patronus Charm that are, even today, widely accepted as true. He proposed that the Patronus was a hidden, but inherent part of one's personality, that only comes to light when the able caster needs it, such as when facing something as inhumanly evil as a Dementor (this, according to Spangle, allows us to explain why Patronuses appear in forms that their casters may not expect, or for which they have never felt a particular affinity).
He also suggested that witches and wizards whose Patronus took the form of their favourite animal were obsessive or eccentric, as they were wizards who could not hide their essential self in common life, in effect parading tendencies that others would prefer to conceal.
- Pottermore (Mentioned only)