 Unification without reduction. Reduction is not the sole standard of unity and models of unification without reduction have proliferated. In addition, such models introduce in turn new units of analysis. An early influential account centers around the notion of interfield theories (Darden and Maull 1977; Darden 2006). The orthodox central place of theories as the unit of scientific knowledge is replaced by that of fields. Examples of such fields are genetics, biochemistry and cytology. Different levels of organization correspond in this view to different fields: Fields are individuated intellectually by a focal problem, a domain of facts related to the problem, explanatory goals, methods and a vocabulary. Fields import and transform terms and concepts from others. The model is based on the idea that theories and disciplines do not match neat levels of organization within a hierarchy; rather, many of them in their scope and development cut across different such levels. Reduction is a relation between theories within a field, not across fields.