The Problem of Universals — Realism, Nominalism, and Abelard’s Conceptualism in Medieval Philosophy

Introduction — Two Shirts and a Hidden Contradiction

This lecture begins not with a historical figure or a technical term, but with a question that looks simple and turns out to be anything but.

Imagine two shirts on a table in front of you. Shirt A is blue. Shirt B is also blue. Anyone who sees them would describe the situation using one of the following two sentences:

• Sentence 1: Shirt A and Shirt B both have the same colour.
• Sentence 2: The colour of Shirt A is similar to the colour of Shirt B.

Most people read these as equivalent — two ways of saying the same thing. They are not. They are logically contradictory. If one of them is correct, the other cannot be.

How Sentence 1 counts the items in the world: If both shirts have the same colour, then there is a single colour — one thing — that both shirts share. In the world, there are therefore three items: Shirt A, Shirt B, and one blueness that they both participate in. That shared blueness is a universal form.

How Sentence 2 counts the items in the world: If the colour of Shirt A is similar to the colour of Shirt B, then each shirt has its own colour. They are similar colours, but they are not the same colour — they are two separate properties. In the world, there are therefore four items: Shirt A, Shirt B, the colour of Shirt A, and the colour of Shirt B. There is no single shared universal. There are only two particular properties that resemble each other.

This is the problem of universals in its most compact form. Is there one blueness shared by both shirts (the realist view)? Or are there just two shirts with two separate but similar properties, and ‘same colour’ is simply our way of describing the resemblance (the nominalist view)? The question seems almost trivial. Its implications run through the entire history of philosophy.

The two positions stated simply: Realism says the world contains universals — one shared form (‘blueness’) that many particular objects participate in. Nominalism says the world contains only particulars — individual objects and their individual properties. Whatever general words we use (‘blue,’ ‘human,’ ‘just’) are names or sounds we apply to groups of similar particulars. Nothing in reality corresponds to those words except the individuals themselves.

1. Why This Question Mattered — The Historical Context

After the extraordinary philosophical work of Eriugena in the ninth century, European philosophy went silent for almost two hundred years. There was no philosophical progress, no original thinking. When philosophy resumed in the eleventh century, it resumed in a completely different spirit.

The earlier approach — building grand, all-inclusive metaphysical systems — had proven dangerous. Eriugena’s system was the clearest example: built with rigour and intelligence, it ended in conclusions the Church could not accept. The new generation of thinkers was more cautious. Instead of building systems, they devoted themselves to two very specific technical problems: the status of universals, and the relationship between faith and reason. Both needed to be resolved before any larger philosophical project could safely proceed.

Why Reason Needed to Be Justified

Medieval thinkers were not doubting that the contents of Christian faith were true. They were entirely confident that what the Bible taught and what the Church proclaimed was the highest truth available to human beings. The question was not whether these truths were true, but whether they could be supported by reason — and if so, how far reason could go in that support.

Two practical pressures made this question urgent. First, contact with non-Christians — through trade, crusades, and increasing cultural exchange — brought Christian thinkers into dialogue with Jews, Muslims, and pagans who did not share their premises. Against such audiences, appeals to the authority of scripture were useless. The only effective tool was rational argument. Second, the problem of heresy: Christian thinkers who followed reason too confidently, like Eriugena, ended up departing from orthodox doctrine. If reason was going to be used, the Church needed to know where its legitimate domain ended.

The central problem:  Before reason could be used to support faith, the nature of reason itself had to be understood. Specifically: what is reason’s proper object — what kinds of things can reason actually grasp? And what is the metaphysical status of those things? The answer to this question led directly to the problem of universals.

Reason, Senses, and Universals

Medieval thinkers recognised three sources of knowledge: revelation (the Bible and Church teaching), perception (the senses), and reason (rational inference and understanding). Revelation was beyond question. Perception was of limited interest — the senses give us knowledge of particular physical objects, but God and the great truths of Christian theology cannot be known through the senses. So the philosophical focus fell entirely on reason.

Now: if the senses give us knowledge of particular things (this tree, this person, this stone), what does reason give us knowledge of? The answer, going back through Augustine and Plato, was: reason grasps universals. ‘Socrates’ refers to a particular person I can see. ‘Human’ refers to something different — something that can be said of Socrates, Plato, Aristotle, and every other human being who has ever lived or ever will live. That something is a universal.

But what exactly is a universal? Where does it exist? Is it real in any genuine sense? Is it in the physical world or outside it? And why does the answer to this question matter for what reason can know, and therefore for what faith can be rationally supported? These were the questions the eleventh-century thinkers had to answer.

2. Porphyry’s Framework — Three Questions and a Fourth

Medieval thinkers did not approach the problem of universals from scratch. They had a text: Porphyry’s Introduction to Aristotle’s Categories, which Boethius had translated from Greek into Latin and commented on extensively. In it, Porphyry posed the question with precision — and then, famously, refused to answer it.

Porphyry wrote that questions about universals ‘are most exalted business and require very great diligence of inquiry’ — and declined to address them in this introduction. But he did specify the questions. His framework gave all subsequent medieval thinkers the structure within which to think:

• Question 1: Do universals exist in reality independently — in the way that Plato’s Forms do — or do they exist only in the understanding, as mental constructions?
• Question 2: If they exist in reality, are they material (physical) or immaterial (non-physical)?
• Question 3: Do they exist separately from sensible objects — in some other world — or do they exist within sensible objects?

Every medieval position on universals can be understood as a specific combination of answers to these three questions. And as we will see, the combination of answers you choose carries major consequences — not just philosophically, but theologically.

Abelard would later add a fourth question that Porphyry had not asked but that his own analysis made necessary:

• Question 4 (Abelard’s own): Does a universal word require actual existing objects to refer to? If all roses in the world ceased to exist, would the word ‘rose’ still have meaning?

This fourth question cuts to the heart of Abelard’s theory of abstraction — we will return to it after examining the earlier positions.

3. Extreme Realism — Universals as Independent Forms

The dominant view among early medieval thinkers — from Eriugena through to William of Champeaux in the eleventh and twelfth centuries — was extreme realism. On this view, universals are genuinely real, independently existing, immaterial entities that exist separately from the particular physical objects that participate in them. This is, essentially, the Platonic theory of Forms translated into a Christian theological context.

In terms of Porphyry’s questions: Yes, universals exist in reality (independently). Yes, they are immaterial. Yes, they are separate from sensible objects.

William of Champeaux — one of the leading teachers in Paris at the time, under whom Abelard briefly studied — held that the same universal is fully and completely present in each individual of a species. Bob, Sabrina, Socrates, and Plato all contain the full essence of humanity. The differences between them (height, hair colour, personality) are minor modifications of their essential humanity — accidental qualities, not differences in substance.

Theological Advantages — Why This View Was Attractive

Extreme realism solved two doctrinal problems that had no easy solution without it.

Original Sin

The theological problem: Adam sinned in the Garden of Eden. His sin corrupted his will. But how can that one act — committed by one person, thousands of years ago — affect the moral constitution of all human beings who came afterward, including people born centuries or millennia later who had no involvement in it? If each human being is a completely separate individual, with no metaphysical connection to Adam, there seems to be no mechanism by which his corruption could reach them.

Extreme realism provides an elegant solution. There is a universal human nature — a single Form called ‘humanity’ — that God created when He created Adam. Adam did not merely corrupt himself; he corrupted the universal human nature of which he was the embodiment. Since every subsequent human being participates in that same universal human nature, the corruption is built into the nature that all humans share. It is not transmitted from person to person like a disease; it is present in the very form of what it means to be human.

The tree and the root: Imagine a tree with many branches, leaves, flowers, and fruit. A single root feeds them all. If someone pours poison into that root, the poison reaches every branch, every leaf, every piece of fruit — not because it travels from one leaf to another, but because everything draws from the same poisoned source. Original sin works the same way under extreme realism. The universal human nature is the root. Adam’s sin poisoned that root. Every particular human being, drawing their humanity from that root, inherits the corruption.

The Trinity

The doctrinal problem of the Trinity is equally difficult: Christianity insists that God is simultaneously one and three — one God, three persons (Father, Son, Holy Spirit). How can three distinct persons each be fully God, while there is only one God?

Extreme realism offers a direct answer. There is one divine essence — one universal form of divinity. The Father, Son, and Holy Spirit are three particular persons who each fully participate in that single divine essence. Just as Socrates, Plato, and Aristotle are three individuals who each fully participate in the universal form of humanity, the three Persons of the Trinity are three individuals who each fully participate in the universal form of divinity. The essence is one; the persons are three. There is no contradiction.

The Fatal Problem — Pantheism

Extreme realism’s theological advantages were real. But its philosophical consequences were fatal. The hierarchy of universals leads somewhere the Church could not accept.

The ascending hierarchy: Start with particular human beings: Socrates, Plato, Aristotle. They all participate in ‘human.’ Humans — along with dogs, dolphins, and donkeys — all participate in ‘animal.’ Animals participate in ‘living being.’ Living beings participate in ‘being.’ And ‘being’ — the most general, most comprehensive, most real of all universals — is the ultimate level. If ‘being’ is the highest universal, and God is the highest reality, then ‘being’ is God. But everything that exists participates in ‘being.’ Therefore: everything that exists is a part of God.

This is pantheism — the doctrine that God and the world are one and the same, that there is no genuine separation between the Creator and the created. It is exactly the conclusion that Eriugena reached in his philosophical system, and exactly the conclusion the Church condemned him for. The early realists — Odo of Tournai, William of Champeaux, and others — arrived at the same position as Eriugena, largely without realising it.

The realists’ blind spot: The early medieval thinkers who adopted extreme realism did not have access to Plato’s Parmenides, or to Aristotle’s detailed criticisms of the Theory of Forms, or to much of Plato’s other work beyond the Timaeus. If they had possessed Aristotle’s logical arsenal, they would have seen the problems with realism much earlier. They also seem not to have read Eriugena carefully — if they had, his pantheism would have been a clear warning.

4. Nominalism — Roscelin and ‘Vocal Winds’

Faced with the pantheistic implications of extreme realism, some thinkers went to the opposite extreme. Nominalism holds that universals are not real things at all. Only individual particular objects are real. Universal words like ‘human,’ ‘animal,’ or ‘horse’ are merely names — sounds we produce with our voices — that we use as shorthand for groups of individuals.

Roscelin, a teacher of logic in France around 1050–1120, was the most prominent advocate of this position. He reportedly called universals ‘vocal winds’ (flatus vocis in Latin) — mere puffs of air that we shape into sounds and use to label collections of individuals. The universal ‘human’ is just the sound ‘human,’ which we have decided to apply to Socrates, Plato, Aristotle, and everyone else in the class. There is nothing in reality — no single thing — that the word ‘human’ names except the individuals themselves.

In terms of Porphyry’s questions: universals do not exist in reality independently. They exist only as sounds in language. They are neither material nor immaterial — they are just labels. They are neither inside particular objects nor in a separate world — they are human linguistic conventions.

Three Problems with Nominalism

While nominalism avoided pantheism, it generated its own set of severe difficulties.

• Original sin becomes inexplicable. If each human being is a fully separate, distinct individual with no shared substance connecting them to Adam, there is no mechanism by which Adam’s moral failure could affect anyone else. Each person is a complete, self-contained unit. Why should I bear the consequences of what a different, completely separate individual did thousands of years ago? Extreme nominalism cannot answer this.
• The Trinity collapses into tritheism. Roscelin himself drew the logical conclusion openly: if there is no common divine essence shared by the three Persons — if ‘God’ is merely a name we use for a collection of individuals — then Father, Son, and Holy Spirit are three separate Gods. The monotheism that Christianity insists upon is simply impossible on nominalist premises. This conclusion got Roscelin condemned as a heretic at the Council of Soissons in 1093. He was obliged to publicly retract his position.
• Universals become arbitrary and subjective. If a universal is merely a name that a group of individuals happens to share, what determines which individuals belong to the group? If nothing real in the world corresponds to ‘human,’ then calling Socrates, Plato, and a stone all ‘human’ would be equally valid — just a different decision about which label to apply to which collection. Universals would be entirely subjective, determined only by our linguistic choices, not by anything in reality. This would make all science, all classification, and all theological argument groundless.

5. Peter Abelard — Life and Philosophical Character

The resolution of this impasse came from Peter Abelard (1079–1142), one of the most brilliant, difficult, and ultimately tragic figures of medieval philosophy.

Abelard was born near Nantes in western France in 1079. His father had planned a military career for him; Abelard abandoned this to pursue what he described as the ‘art of disputation’ — formal philosophical argument. He travelled to Paris, where the leading teacher of the day was William of Champeaux, a committed extreme realist. Abelard studied under him, found his theory philosophically inadequate, and said so — publicly and without restraint. He developed arguments that forced Champeaux to revise his position repeatedly. When Champeaux would not change enough, Abelard set up his own rival school and drew away Champeaux’s students.

Abelard’s life beyond philosophy was equally dramatic. While teaching in Paris he fell deeply in love with a young woman, Heloise, the niece of a canon of Notre Dame. When a son was born to them, Heloise’s uncle arranged a secret marriage. When this became publicly known, the uncle had Abelard attacked and mutilated in his sleep. Heloise entered a convent; Abelard entered a monastery. Despite their permanent separation, they maintained a correspondence throughout the rest of their lives — letters of great philosophical and personal depth that have survived as one of the most remarkable documents of the medieval period.

Church opposition followed Abelard wherever he went. He was condemned for heresy more than once. His writings were ordered burned. He died in 1142, still under the shadow of condemnation from the Council of Sens (1141). His philosophy did not die with him.

Abelard’s philosophical method was the prototype of Scholasticism — the style of thought that would dominate the medieval period. He worked through texts sentence by sentence, extracting their precise logical meaning, testing each claim against others, refusing to accept anything on authority alone if it contained a contradiction. He was the first major medieval philosopher who genuinely thought for himself rather than commented on authorities.

6. Abelard’s Attack on Extreme Realism — Two Arguments

Abelard had studied not only under William of Champeaux but also under Roscelin. He saw the problems in both positions. He began his constructive work by demolishing the extreme realist view with two precise logical arguments.

The Deeper Problem: Substances Cannot Have Opposite Properties

The second argument is the more philosophically striking of the two. Let us walk through it step by step, because the logic is exact.

Extreme realism says: the same universal substance ‘animal’ is fully and equally present in both Socrates and a donkey. Both are animals; both participate in the same universal animal-substance.

Now: Socrates is a rational being. His rationality is part of what he is. The donkey is an irrational being — its irrationality is part of what it is. Both participate in the universal ‘animal.’

Therefore: the universal substance ‘animal’ is, in Socrates, the bearer of rationality. And the same universal substance ‘animal’ is, in the donkey, the bearer of irrationality. The same substance, in the same world, at the same time, is both rational and irrational.

But rationality and irrationality are logical opposites. A single substance cannot be both rational and irrational simultaneously. This is not merely counterintuitive — it is a violation of the most basic law of logic: the law of non-contradiction. Nothing can be both X and not-X at the same time.

The argument compressed: If the same substance (‘animal’) is fully in Socrates AND fully in the donkey, then since Socrates is rational and the donkey is irrational, the same substance is both rational and irrational. This is a contradiction. Therefore the same substance cannot be fully in both. Therefore extreme realism — which requires it to be — is false.

7. What Universals Are — Abstraction and Concept Formation

Having shown that universal things do not exist — that there is no single universal ‘human’ present in multiple individuals — Abelard does not conclude that universals are nothing. He makes a distinction that is philosophically crucial: there are no universal things, but there are universal words; and those words are not empty. They have meaning. The question is: what do they mean, and how do they acquire that meaning?

The Power of Abstraction

Abelard introduces the concept of abstraction as the key to understanding universals. Abstraction is the mind’s power to attend to one specific aspect of an object in complete isolation from all its other aspects.

How abstraction works: In front of you is a black keyboard and a black mouse. Your mind can attend exclusively to their colour, setting aside everything else — their shape, their weight, what they are made of, what they are for, their size, their texture. You are not ignoring these other properties in the sense of forgetting they exist; you are simply not attending to them right now. You are focusing your attention on one aspect only. This is abstraction: the selective focus of the mind on one aspect of a thing.

Abelard now combines this with a second observation: many different things can share similar aspects. The keyboard is black. The mouse is black. Different objects, same colour. The mind, using its power of abstraction, attends to the colour of each object separately — and notices that they are similar. The similarity is not in our heads; it is a genuine feature of the two objects. Both really are black. The likeness between them is real.

From this genuine, real similarity, the mind forms a concept. It calls that concept ‘black’ or ‘blackness.’ The concept is a mental entity — it exists in the mind, not in the world separately from the objects. But it is not invented or arbitrary: it tracks something real.

The Concept — Not an Empty Name, Not an Independent Form

This is Abelard’s middle path, and it is precise:

• Against extreme realism: There is no universal ‘blackness’ floating in a Platonic world above and beyond the physical black objects. The universal exists only as a concept in the mind. There is no third world of Forms.
• Against nominalism: The concept ‘black’ is not a random label we could have applied to anything. It is grounded in a real similarity between real objects. When I say ‘black,’ I am pointing at something in the world — the objective colour-property that those objects genuinely share. The concept is not empty.

Abelard describes the concept formed by abstraction as something like a ‘common and confused image’ — a general mental impression that captures what is similar in many individuals without specifying what is particular to any one of them. ‘When I hear man,’ he writes, ‘a certain figure arises in my mind which is so related to individual men that it is common to all and proper to none.’ It is common to all (it captures what all men share) and proper to none (it does not describe any specific man).

Think of what happens when you squint your eyes at a person in the distance. You can tell there is a human being there, but you cannot identify them — you can see the general human shape but not the distinguishing features. The universal concept ‘human’ is something like that: a general image, common to all individuals of the kind, proper to none.

8. The Three Positions Compared

With all three positions now in view, it is worth setting them side by side clearly. The following table covers all the major dimensions of comparison:

9. Abelard Answers Porphyry — All Four Questions

The most philosophically interesting of the four answers is the last. Abelard’s fourth question — what happens to the universal if all its instances are destroyed? — reveals something important about the nature of conceptualism.

The rose that has disappeared: Suppose all roses in the world were to disappear overnight. Every single rose — every plant, every petal, every seed. Would the word ‘rose’ still have meaning? Abelard’s answer: yes, as a concept retained by minds that formed it when roses existed. The concept persists in memory and understanding. But the word would no longer refer to anything actual in the world. It would be meaningful but empty of current reference.

The dinosaur: Dinosaurs no longer exist. Yet ‘dinosaur’ is a meaningful word — we know what it means, we can use it in true sentences (‘the T-Rex was a dinosaur’), we can reason about it. The concept is in our minds, formed from the evidence of fossils and bones. But it refers to nothing currently living. The concept is real and meaningful; its referent has been destroyed. This is exactly the situation Abelard describes: the universal ‘rose’ would be meaningful as a concept but would lack any actual referent in the world.

10. How Conceptualism Works — The Two Requirements

Abelard’s conceptualism makes the existence of a universal concept depend on two things, both of which must be present:

• First: Real objects with genuinely similar properties must exist in the world. You cannot form the concept ‘black’ if there are no black things. The similarity is not constructed by the mind — it is found in the world. Without the real similarity to track, the concept would have no grounding.
• Second: A mind that can observe those objects and abstract the similarity must exist. Even if a million black objects exist, the concept ‘black’ does not exist unless some mind attends to them, notices their similarity, and forms the concept. Without the abstracting mind, there is no universal.

Both are necessary. Neither alone is sufficient. This is what distinguishes conceptualism from both its rivals:

• Realism: The Form of Blackness exists whether or not any black objects exist, and whether or not any minds exist. Neither is required. The Form is entirely independent.
• Nominalism: Only the individual black objects are needed — plus the decision to group them under a label. No genuine similarity in the world needs to be tracked; no real basis for the grouping is required.
• Conceptualism: Both are required. Objects with a real, objective similarity + a mind that abstracts from that similarity = a universal concept.

This is also why, under conceptualism, if all black objects disappeared, the concept ‘black’ could not be freshly formed. You can retain the concept from memory, but you cannot form it anew. And if there were no minds at all, no universal concepts would exist even if the world were full of similar objects.

11. The Historical Significance of the Debate

The problem of universals was not merely a medieval puzzle about words and Forms. It was the central conceptual obstacle that had to be cleared before the great synthesis of the thirteenth century — Thomas Aquinas’s integration of Aristetelian reason with Christian theology — could be built.

The problem was also not solved by Abelard. His conceptualism showed the direction — the mind forms universals by abstracting from genuine similarities in things — but it left the account incomplete. What exactly is the similarity that grounds the concept? What is its metaphysical status? Abelard’s psychology was, as commentators noted, ‘certainly sketchy.’ He identified the right question to ask; he did not fully answer it.

The full answer came when Aristotle’s complete works were recovered through the Islamic world and made available in Latin translation in the twelfth and thirteenth centuries. Aristotle had, fifteen centuries earlier, developed exactly the kind of theory that medieval thinkers needed: forms are not independent Platonic entities, but neither are they mere names. They are properties of individual things, which the mind can abstract and consider in general terms. This is the position that Thomas Aquinas would synthesise into a coherent and complete philosophy.

The scope of the problem did not contract when it moved into modern philosophy — it expanded. In analytic philosophy, the same questions appear as: do numbers exist? Do sets exist? Do natural kinds exist? Are properties real? Every time a discipline asks whether its basic entities are ‘real’ in some robust sense or merely conventional categories, it is asking a version of the medieval problem of universals. The problem appears in logic (predicate logic and quantification over properties), in the philosophy of mathematics (Platonism vs formalism vs nominalism about numbers), in the philosophy of science (realism about natural kinds), and in contemporary metaphysics (trope theory, nominalism, sparse vs abundant theories of properties).

The problem is not solved. It remains live across multiple philosophical disciplines. What medieval philosophy contributed was the sharpest early formulation of the question, the clearest identification of the three main options and their consequences, and the most important step toward a middle path — Abelard’s conceptualism — that pointed the way to Aristotle’s more developed solution.

Conclusion

The problem of universals looks, on the surface, like a technical dispute about the meaning of words. It is not. It is a question about the structure of reality itself: are there general features of the world — kinds, properties, types — that exist in some genuine sense, or is reality nothing but a swarm of individual particular objects?

The medieval debate forced the answer into sharp focus. Extreme realism (yes, Forms exist independently) was theologically useful but philosophically led to pantheism. Nominalism (no, only particulars exist; universals are mere sounds) was philosophically clean but theologically disastrous and epistemologically absurd. Something in between was needed.

Abelard found that middle path. His two arguments against extreme realism were precise and devastating. His account of abstraction — the mind’s power to attend to genuine objective similarities in things and to form concepts from them — gave philosophy its most sophisticated medieval answer to the problem of universals. And his fourth question — what happens to the universal when its instances are gone? — drew attention to the dependence of universal concepts on two things: real similarities in the world, and minds capable of noticing them. That answer was not final. Aquinas would complete it by drawing on the full resources of Aristotle’s philosophy. But Abelard made Aquinas possible — by clearing away the extreme positions and identifying the conceptual territory in which a genuine solution could be found. Half the task of solving any problem is discovering the right question to ask. Abelard discovered it.

Frequently Asked Questions