The Stanford Encyclopedia of Philosophy (2013; substantive revisions 2018). Edward N. Zalta, ed.

Abstract: According to trope theory, the world consists (wholly or partly) of ontologically unstructured (simple) abstract particulars or, as they are normally called, tropes. Tropes are abstract yet they are not universal, they are particular yet they are not concrete. In accepting the existence of entities characterized in this (unusual) way, the theory can be said to occupy a middle position in between classical nominalism—according to which all there is are concrete particulars—and classical realism—according to which there is a separate and fundamental category of abstract universals. And this, it has been argued, means that trope theory avoids well-known problems with both of those views. By accepting the existence of abstract entities (like shapes and weights), the trope theorist is able to explain how distinct concrete particulars can be simultaneously similar to, and different from, each other. And this is something the classical nominalist, whose basic ontology is more coarse-grained, has been accused of not being able to do (for a famous instance of this critique of classical nominalism, see Armstrong 1978). And by not accepting the existence of universals, she avoids having to accept the existence of a kind of entity many find mysterious, counterintuitive, and “unscientific” (Schaffer 2001: 249f.; Molnar 2003: 22–25; and Armstrong 2005: 310). Apart from this very thin core assumption—that there are tropes—different trope theories need not have very much in common. Most trope theorists (but not all) believe that there is nothing but tropes. Most trope theorists (but, again, not all) hold that resemblance between concrete particulars is to be explained in terms of resemblance between their respective tropes. And most (but not all) hold that resemblance between tropes is determined by their primitive intrinsic nature. In fact, even to call one’s posits “tropes” is by some considered problematic (see especially Bacon 2011). In this entry, different views on the nature and individuation of tropes, on how tropes relate to both universals and concrete particulars, and on how tropes might or might not be used to (dis)solve well known problems in philosophy, are introduced.

Link to published paper here.