Classifying people as others also classifies the classifier. Ancient medical treatises had a variety of different ways of classifying and grouping illnesses. Milling also refers to the process of breaking down, separating, sizing, or classifying aggregate material. They are responsible for classifying and appraising commercially imported merchandise that enter the U.S. Algebraic homology remains the primary method of classifying manifolds. Other theorists developed a plethora of methods for classifying goods. States apply varied methods of classifying registrants. There are many different methods of classifying musical instruments. They offered definitions of the human being and schemes for classifying types of humans. There are several methods of classifying exploits. There is a controversy regarding race as a method for classifying humans. A unified taxonomy (a universal system for classifying viruses) has been established. Paul Topinard developed the as a method of classifying ethnic groups. Criteria for classifying an event as a miracle vary. Several systems for classifying the individual diatom species exist.