dirt is matter out of place I

All of this thinking about dirt makes me kind of numb! "matter out of place" here, "matter out of place" there! The problem is that I can't figure out what the real problem of this definition is: "dirt is matter out of place!" (Mary Douglas).

I guess the main problem I have with it, is that you cannot turn around the defintion: matter out of place is not necessarily dirt! E. g. a fork in a bed is certainly out of place, but is it therefore really dirt? I don't think so!