This chapter outlines one of the medieval philosopher’s favorite solutions to the problem of the existence of universals. It looks at the nominalism-realism issue. This issue was raised in the third century by the Syrian philosopher Porphyry. The history of ontology is the battle over the existence of the world. Naturalists deny that there are abstract entities. Ontologists insist that there are such things, and they try to describe the structure of the world of abstract entities ever more accurately. The battle over the world is the battle over abstract things. This battle involves a fight about the nature of properties: are properties abstract or are they concrete? The chapter discusses this question directly. A certain view about the nature of properties has had a grip on the minds of many philosophers. According to this view, the whiteness of billiard ball A is not the same thing as the whiteness of billiard ball B.