||Universals, in philosophy, are the features or qualities which many objects may share. Redness and squareness are both universals, since they are features or qualities which many objects share: all red objects possess the former universal, all square objects possess the latter.
Particulars are the things or objects which possess universals. This pillar box is a particular and so is that scarf, and they both possess the universal redness: they are both instances of the universal redness.
Realists about universals hold that universals have a reality distinct from their instances that universals exist distinct from the particulars which possess them. Platonists treat universals as having an existence independent from their instances: not merely are there things which possess the quality of being red, but the universal redness also exists. Advocates of this view can thus hold that those universals of which there are no instances nevertheless exist. The universal leprechaunhood exists, even though there is no particular individual which possessesâ€”it even though there are no leprechauns.
Conceptualists hold that universals are in the first instance ideas or concepts in the mind. Unlike realists, they hold that universals have no reality or existence independent of the mind. We have an idea or concept of redness and any object which resembles our idea, or satisfies our concept of redness, has the universal redness. One problem here is that it is seemingly arbitrary which ideas or concepts minds form in the first place. We group together certain objects as red because they resemble our idea or satisfy our concept of redness, but the concept we form is not constrained by any universals existing independently of particulars or by any resemblances between particulars.
Nominalists claim that objects which possess the same universal do resemble each other. Universals do not exist independently of their instances. But it is not simply arbitrary that we group certain things together as being red. Rather, we group certain things together as being red because they resemble each other in respect of their colour. AJ
Further reading D. Armstrong, Universals and Scientific Realism; , B. Russell, Problems of Philosophy.