O problema dos universais talvez seja “o” problema filosófico por excelência. Ted Sider, professor da NYU e autor de diversos livros sobre Ontologia, faz no seu artigo de 2006, além da exposição das suas próprias posições e do tratamento do problema dos “bare particulars”, uma excelente síntese das soluções tradicionais:
The complaint is aimed at the substratum theory, which says that particulars are, in a certain sense, separate from their universals. If universals and particulars are separate, connected to each other only by a relation of instantiation, then, it is said, the nature of these particulars becomes mysterious. In themselves, they do not have any properties at all. They are nothing but a pincushion into which universals may be poked. […] Against substratum theory there is the bundle theory, according to which particulars are just bundles of universals.
The major division my proposed classification recognizes is a division of ontologies into monocategorial and polycategorial ontologies. A monocategorial ontology is an ontology that implies that there is only one primary ontological category — that there is only one ontological category that is not a subcategory of any other ontological category,—and that everything belongs to that category. That is to say, a monocategorial ontology implies that the universal class is an ontological category. A polycategorial ontology, of course, implies that there are two or more primary categories.