Universals
Properties shared by many particulars; the central problem dividing realists, nominalists, and conceptualists.
In practice
Two products are described as premium. Whether that names a real shared property or is a label applied to loosely similar things decides whether the category can support a prediction.
The common mistake
Assuming nominalism is the modest position. Denying universals requires explaining resemblance without appealing to anything shared, which has proved at least as difficult as the alternative.
Universals are the properties or kinds that many particular things can share. Two red apples have redness in common; the question is what that common possession amounts to.
The positions
Realism holds that universals exist in their own right. Plato gave the strongest form in the theory of forms: the form of redness exists outside space and time, and particular red things participate in it. This explains how the same property can be wholly present in many places and pays for it with an additional realm.
Aristotle held that universals are real and exist only in the particulars that have them. There is no redness apart from red things, and the mind abstracts the universal from the instances. This keeps the explanatory work and drops the separate realm.
Nominalism holds that only particulars exist. "Red" is a name we apply to things that resemble each other, and nothing further is shared. William of Ockham argued that positing universals multiplies entities without need. The objection is that resemblance itself looks like a universal, which threatens a regress.
Conceptualism places universals in the mind as general concepts formed from experience, which explains our ability to classify while leaving open why the world cooperates.
Why the argument matters
The dispute decides what a scientific law describes. If laws relate universals, they state something about the structure of reality. If only particulars exist, a law is a summary of how things have behaved so far, which brings Hume's problem of induction directly into view.
The problem of universals was the central controversy of medieval philosophy and remains a live question in ontology.
Concept web
Open the full webQuestions
What is the problem of universals?
The question of whether properties shared by many things — such as redness or being human — exist in their own right, exist only in particular things, or are merely names we apply to things that resemble one another.
What is the difference between realism and nominalism about universals?
Realism holds that universals genuinely exist. Nominalism holds that only particular things exist and that general terms are names for groups of resembling particulars.
How did Aristotle differ from Plato on universals?
Plato held that forms exist independently of the things that instantiate them. Aristotle held that universals are real but exist only in particular things, abstracted by the mind from experience.