Universals are objects in metaphysics that describe common features or relationships between physical objects. For example, the idea of “blue” is a universal. The ocean, the sky and blueberries have the property of being blue, but we have an idea of the universal blueness without reference to a particular object. Other examples include humanity, goodness or beauty: we have instances of things that are or seem human, good or beautiful, but we also have a notion of what humanity, goodness or beauty are.
Universals are contrasted with particulars. Particulars are individual objects which refer only to one individual thing. For example, a specific tree is a particular. It is also an instance of each universal which it resembles, which may include treehood, planthood, greenness, bigger-than-ants–ness, smaller-than-stars–ness — essentially all of the properties, classifications and relations which apply to that particular tree but also to other things which resemble that tree in some way.
Universals can generally be divided into three categories: types (or natural kinds), properties and relations. Types are classifications of objects, such as trees, tables or planets. Properties are aspects of objects, such as size, shape and colour. Relations are descriptions of objects that make reference to other objects, such as “smaller than” or “mother of”.
In metaphysics, it is depated whether or not universals can truly be considered to exist, and if so, where they belong in an ontology. For example, it can be said that there are humans, that is, things that can be classified under humanity — but can it be said that humanity, or humanness, exists? Similarly, many things are red, but does redness exist as an onotlogical object in itself? Similarly, would there be such a thing as redness were there no objects that were red?
There are two main positions on the problem of universals: realism and nominalism. Realism holds that universals actually exist, while nominalists or anti-realists hold that they do not.
Perhaps the most famous realist was Plato, who believed that universals, which he called the Forms, were actual things. Universals do not exist in the same mode of existence as physical particulars for Plato, but instead exist in the realm of the Forms, a sort of heavenly, detached reality. In fact, Plato held that the Forms were more real than the physical particulars we encounter, for the Forms were timeless and unchanging, wheras particulars in the physical world were temporary and corruptable. Plato described things as participating in a Form; for example, a beautiful sculpture participates in the Form of beauty. Plato’s theory of the Forms is most clearly spelled out in his Republic, though he discusses the topic in other works.
Since Plato, the idea of a realm of the forms has become somewhat unpopular. Realists generally consider universals to be things-in-themselves, but deny that they have a place in space and time. According to modern realists ask questions about where the universals exist is, at best, a category mistake. At worst, it is a meaningless utterance.
Some realists say that universals have ontological being, but that being is divided into more than one category. Physical beings exist, wheras non-physical beings, such as universals, subsist in a way that makes them fully independent entities but not existing in the physical world. In either case, realists often hold that universals are mind-independent — that is, they are not
Some philosophers with religious metaphysical assumptions consider universals to exist as thoughts within the mind of God.
Nominalists generally hold that universals do not really exist.
The nominalist response to realist claims is often to ask the question, where do universals exist? Plato believed that unviersals existed in a realm of the forms, whereas modern realists claim that these objects do exist, but have no spatial or temporal properties. Nominalists reject this answer, either on the claim that we cannot suppose the existence of things without spatial or temporal properties since we do not have evidence of their existence, or on the claim that we do not need to posit their existence in order to explain our experience — that is, an ontology without universals is not deficient in any way. Some ontologists, most notably Quine, advocate for minimalizing ontology as much as possible, only accounting for what is necessary to explain experience.