In other words, universals are repeatable or recurrent entities that can be instantiated or exemplified by many particular things.
The problem arises from attempts to account for the phenomenon of similarity or attribute agreement among things.
Taking "beauty" as an example, four positions are: Taking a broader view, the main positions are generally considered classifiable as: extreme realism, nominalism (sometimes simply named "anti-realism" with regard to universals),[6] moderate realism, and idealism.
Extreme Realists posit the existence of independent, abstract universals to account for attribute agreement.
Particulars are the referents of proper names, such as "Phaedo," or of definite descriptions that identify single objects, such as the phrase, "that person over there".
Examples include by D. M. Armstrong, Nicholas Wolterstorff, Reinhardt Grossmann, Michael Loux.
Nominalists hold that universals are not real mind-independent entities but either merely concepts (sometimes called "conceptualism") or merely names.
The ness-ity-hood principle is used mainly by English-speaking philosophers to generate convenient, concise names for universals or properties.