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Particulars are things that exist in only one place at a time (this apple, you); universals are things that can be wholly present in multiple places simultaneously (redness, triangularity). The problem of universals asks whether properties are genuine universals (realism) or whether only particulars exist and talk of shared properties is reducible or fictional (nominalism). Realists argue universals explain the resemblance between distinct red things; nominalists find universals ontologically extravagant and seek substitutes. This debate connects directly to questions about causation, natural laws, and scientific explanation.
Work through Armstrong's Universals: An Opinionated Introduction alongside a nominalist response such as Lewis's papers. Try to construct the strongest argument for each side before comparing.
When you studied substance and property, you learned that objects have properties — this apple is red, round, and sweet. The problem of universals pushes further: what exactly is a property? When we say this apple and that fire truck are both red, are we saying they share something — a single entity, redness, present in both places at once? Or are we just applying the same word to similar but ultimately distinct things? The first answer is realism about universals; the second is nominalism.
Particulars are things that exist in only one place at a time. You, this apple, and the Eiffel Tower are particulars — each is wholly located in a single spatial region and cannot simultaneously be wholly present elsewhere. Universals, on the realist view, are fundamentally different: redness, triangularity, and mass are entities that can be wholly present in multiple locations simultaneously. When you see two red things, the realist says the same entity — redness — is present in each.
The main argument for realism is that universals explain resemblance. The 'one over many' problem asks: in virtue of what are two distinct red things both red? The realist answer is that they share a universal. This gives similarity an objective grounding. Without universals, says the realist, resemblance becomes unexplained or circular — you would need to say two things are similar just because they are, which explains nothing.
Nominalists find this price too high. Positing abstract entities that somehow exist in multiple places strikes many philosophers as ontologically extravagant and mysterious. Nominalists propose alternatives: resemblance nominalism says redness is just a class of things that sufficiently resemble a set of paradigm cases; predicate nominalism says 'red' is just a predicate we apply, with no corresponding entity in the world. Note carefully that nominalists are not denying that objects share features in ordinary language — they are giving a deflationary account of what those shared features amount to metaphysically.
This debate connects to questions far beyond color. Whether there are universals bears on the status of natural laws (are they universal connections between properties?), mathematical objects (are numbers universals?), and causation (does one event cause another because they instantiate universal causal relations?). Getting clear on the distinction between particulars and universals is foundational for the rest of metaphysics.
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