Problem of Universals

Historical Background

The Problem of Universals concerns the status of general properties, kinds, and relations—entities apparently shared by many particular things. When we say that two apples are both red, or that Socrates and Plato are both human, we seem to attribute something identical—“redness,” “humanity”—to different individuals. The philosophical problem is to explain what, if anything, this shared element is and how it exists.

In Plato, the problem takes its classical form. Plato posits Forms (e.g., the Form of Beauty, the Form of Justice) as mind-independent, non-spatiotemporal universals that particular things “participate in” to possess their properties. This is often called Platonic realism or ante rem realism, since Forms exist “before” or independently of their instances.

Aristotle modifies this approach. He accepts that there are universals (like “human” or “white”), but holds that they exist in particular things, not in a separate realm. This is Aristotelian realism or in re realism, positing universals as immanent in objects rather than transcendent.

The problem became central in medieval scholastic philosophy, where it structured disputes among thinkers like Peter Abelard, Thomas Aquinas, Duns Scotus, and William of Ockham. They debated whether universals are real entities, mental constructs, or mere words. The issue remains important in contemporary metaphysics and philosophy of language, where it connects to questions about natural kinds, properties, and the semantics of general terms.

Main Positions

Philosophers commonly classify responses into several broad camps: realism, nominalism, conceptualism, and related variants.

Realism about Universals

Realists hold that universals genuinely exist and are somehow shared by multiple particulars.

• Platonic (ante rem) realism: Universals exist independently of space, time, and particular things. “Redness” or “triangularity” are abstract entities; red objects are red because they participate in, or exemplify, the universal Redness. This view explains exact sameness of property but faces questions about how particulars “access” or relate to a non-spatiotemporal realm.

• Aristotelian (in re) realism: Universals exist only in their instances. There is no separate realm of Forms; rather, the universal “humanity” is wholly present in each human individual. This promises a closer connection to empirical reality but has to explain how the same universal can be “wholly” present in multiple places at once.

Realists often argue that universals are needed to ground:

• the truth of general statements (“All electrons have charge −1e”),
• objective similarities and laws of nature, and
• the distinction between natural and merely accidental groupings.

Nominalism

Nominalists deny the existence of universals as real entities. On nominalist views, only particular things exist; generality is handled in other ways.

Major forms include:

• Predicate nominalism: To say that things are both red is merely to say that the predicate “is red” is truly applied to each. There is no additional metaphysical entity “redness” shared by them.

• Class or set nominalism: General terms correspond to sets or classes of particulars (e.g., the class of all red things), not to universals. This moves the work of generality into set theory but faces the question whether sets themselves are universal-like abstract entities.

• Resemblance nominalism: Instead of positing universals, this view explains generality via networks of resemblance among particulars. Things are all “red” because they resemble paradigmatically red things to a sufficient degree.

Nominalists generally seek ontological parsimony, avoiding a realm of universals. Critics, however, argue that nominalists still owe explanations of exact similarity, lawlikeness, and meaning that can be hard to provide using only particulars and linguistic or set-theoretic tools.

Conceptualism and Moderate Positions

Conceptualists hold that universals exist, but only as contents of thought—as concepts in the mind rather than as external entities. On this view, different particulars are grouped under one general term because our cognitive capacities organize them under a single concept (e.g., the concept of “triangle”). Medieval thinkers like Abelard and some readings of Ockham are often classified as conceptualists.

A related position is moderate realism, often associated with Aquinas, which maintains that universals have a basis in the real features of things (following Aristotle), but emphasizes that their universal character (as opposed to their being particularized in this or that thing) is realized in the intellect’s act of abstraction.

Key Arguments and Challenges

The Problem of Universals is sharpened by a set of interconnected puzzles and argumentative pressures.

The One-over-Many Problem

At the core is the one-over-many issue: how can there be one property shared by many things? For realists, this is an argument for universals:

1. Distinct objects (e.g., several red apples) are all correctly described as “red.”
2. If the same property is truly predicated of each, there must be something the same in them.
3. That “something the same” is a universal (Redness).

Nominalists and conceptualists typically reject premise (2) as requiring a shared entity, instead appealing to shared linguistic usage, conceptual classification, or resemblance patterns.

The Problem of Resemblance

Nominalists, especially resemblance nominalists, must explain why resemblance seems to come in kinds (color resemblances, shape resemblances, etc.):

• If resemblance itself is a universal relation (Resemblance), nominalism arguably reintroduces universals.
• If there is a family of resemblance relations, nominalists must explain what unifies them without appealing to further universals.

Realists contend that universals provide a straightforward reason why things resemble each other: they literally share the same property. Nominalists respond that positing universals may be metaphysically costly and that resemblance (or resemblance plus other resources) may be primitive.

The Problem of Abstract Reference and Truth

Another cluster of arguments focuses on language and truth:

• General terms like “red,” “electron,” or “justice” appear to refer to something common across instances.
• Scientific laws and mathematics seem to mention properties and relations (e.g., mass, charge, equality).

Realists argue that the best explanation of the truth and objectivity of such discourse is that these terms refer to real universals. Anti-realists claim that such talk can be treated as systematically paraphrasable, or that reference to universals is not required for explanatory adequacy.

Parsimony and the “Ontological Cost”

Nominalists often invoke Ockham’s razor: do not multiply entities beyond necessity. They argue that:

• We can describe and predict the world effectively using only particulars plus linguistic or conceptual practices.
• Introducing universals yields a more crowded ontology that may not yield sufficient theoretical benefits.

Realists reply that explanatory power and theoretical unification sometimes justify a richer ontology. If universals are needed to account for objective similarity, laws of nature, or the semantics of general terms, then their positing is not gratuitous.

The Problem of Universals therefore serves as a focal point for broader questions about what exists, how language and thought latch onto the world, and how to balance ontological economy against explanatory adequacy. No consensus solution has emerged, and the debate continues to shape contemporary metaphysics, philosophy of language, and philosophy of science.