@book { GeistigeWelten,
    title = "Geistige Welten",
    subtitle = "Aufsätze aus vierzig Jahren",
    author = "Franz {Pöggeler}",
    year = "2006",
    publisher = "Peter Lang Verlag",
    address = "Berlin, Germany",
    url = "https://www.peterlang.com/document/1101028"
}