Tönnies drew a sharp line between the realm of conceptuality (of sociological terms, including "normal types") and the realm of reality (of social action). The first has to be treated axiomatically and in a deductive way (Pure Sociology), the second, empirically and in an inductive way (Applied Sociology). Following Tönnies, reality (the second realm) cannot be explained without concepts, which belong to the first realm, or else you will fail because you try to define x by something derived from x.
Tönnies' Normaltyp was hitting at the German sociologist Max Weber, whose "ideal type" (Idealtyp) was coined as an "accentuation" of certain elements of a real social process, which is under sociological (or historical) scrutiny. Nevertheless, Weber's term survived in the sociological community, since his Idealtyp helped to understand social forces, and for him "sociology" had both to explain and to understand things - a daring combination, but successful in the eyes of many sociologists.
