Filch is revealed to be a Squib in "Harry Potter and the Chamber of Secrets" when Harry accidentally discovers that he is trying to teach himself basic magic from a "Kwikspell" correspondence course. Nonetheless, Filch is at least able to use wizarding devices that have their own innate magic, such as the Secrecy Sensor used in "Harry Potter and the Half-Blood Prince". It is hinted that his dislike of students may stem from his disappointment and jealousy at not being able to do magic. In "Harry Potter and the Deathly Hallows", the final book, when the school starts preparing itself for the Battle of Hogwarts, Filch is seen yelling that students are out of bed. He is later seen overseeing the evacuation of younger students.