Concepts of materials are formed by observing the differences in the
constituent materials of entities. (Materials exist only in the form of
specific entities, such as a nugget of gold, a plank of wood, a drop or an
ocean of water.) The concept of “gold,” for instance, is formed by isolating
gold objects from all others, then abstracting and retaining the material, the
gold, and omitting the measurements of the objects (or of the alloys) in which
gold may exist. Thus, the material is the same in all the concrete instances
subsumed under the concept, and differs only in quantity.