First, mechanists criticize the idea that reduction should be understood primarily as a relationship between theories. In integrating their results, scientists are not simply building theories simpliciter; they are building theories about mechanisms. Mechanisms can perhaps be described using formal accounts of theories—perhaps they can be axiomatized in predicate logic or reconstructed as set theoretic predicates. But such formal accounts of the structures of scientific theories gloss over the mechanistic structures crucial for understanding how these theories are constructed and evaluated (Craver 2001b).