When Do You Step In?

We had a discussion in class about eccentrics based on the documentary Home Movie. At what point should authorities get involved in someone's life? Two of the people featured lived in a home with 11 cats and designed cat platforms and runways in every room of their house. They were able to function but what about the people with 100 cats?

If I start collecting people and keeping them in a pit under my garage, it might be reasonable for police to intervene but what if I'm into self-mutilation? At what point is someone harmful to himself and/or others?