To understand concepts, and to protect them from those who commit the stolen concept fallacy, one must respect the hierarchical nature of knowledge. Keeping concepts tied to reality requires a process of reduction. But this process can be very difficult, largely because higher-level concepts, such as "value", are abstract and complex.

This lecture focuses on the role of definitions in reduction and thus provides some pointers on how to reduce knowledge. We will see why Ayn Rand said in Introduction to Objectivist Epistemology that "[d]efinitions preserve, not the chronological order in which a given man may have learned concepts, but the logical order of their hierarchical interdependence."