Nominalism and Realism are the two most distinguished positions in western metaphysics dealing with the fundamental structure of reality. According to realists claim that all entities divide up into two major groups: particulars and universals; Nominalists instead argue that there are only particulars.
The Resemblance Question
Resemblance is crucial to both sort out things (e.g. dogs vs cats, apples vs pears, red things vs blue things …) and to explain regularities in patterns of events (e.g. weather forecast, developmental trajectories, dietary patterns, people’s behaviors …) But, in virtue of what do we claim that two or more things resemble each other? This is what I call the Resemblance Question.
Now, it is important to realize that the resemblance question is tied to questions regarding the identity of things. Resemblance is cognate to identity in that, if two things resemble each other, typically, this is because they are identical under some qualitative respect.
Here it is important to keep distinct two types of identities. On the one hand we have qualitative identity, as when we say that both Socrates and Plato are wise; on the other we have numerical identity, as when I say that I am the same as fifteen years ago or that that’s the same guy I saw yesterday stealing candies from the jar in the common room. Numerical identity judges of identity between particulars; qualitative identity establishes identities between qualities.
Realism offers a solution to the Resemblance Question by postulating the existence of two kinds of entities, particulars and universals: particulars resemble each other because they share universals (incidentally, universals can resemble each other as well by sharing other universals – e.g., wisdom and generosity resemble each other in that they are both virtues.) Among the most famous realists, Plato and Aristotle.
The intuitive plausibility of realism is evident. Realism allows us to take seriously the subject-predicate structure of discourse through which we represent the world. When we say that Socrates is wise it is because there are both Socrates (the particular) and wisdom (the universal) and the particular exemplifies the universal.
Realism also can explain the use we often make of abstract reference. Sometimes qualities are subjects of our discourse, as when I say that wisdom is a virtue or that red is a color. The realist can interpret these discourses as asserting that there is a universal (wisdom; red) that exemplifies another universal (virtue; color).
Nominalists offer a radical solution to the resemblance question: there are no universals, only particulars. The basic idea is that the world is made out of particulars and the qualities are of our own making: they stem from our representational system (the way we cognize the world) or from our language (the way we speak of the world). Because of this, nominalism is clearly tied in a close manner also to epistemology
The most distinguished nominalists include Medieval philosophers William of Ockham (1288-1348) and John Buridan (1300-1358) as well as contemporary philosopher Willard van Orman Quine.
Problems For Nominalism and Realism
From the millenary debate between supporters of those two opposed camps spurred some of the most puzzling problems in metaphysics, such as the puzzle of the ship of Theseus, the puzzle of the 1001 cats, and the so-called problem of exemplification (that is, the problem of how particulars and universals can be related to each other.) This is what renders the debate regarding the fundamental categories of metaphysics so challenging and fascinating.
Further Online Readings
The entry on Nominalism in Metaphysics at the Stanford Encyclopedia of Philosophy.
The entry on Platonism in Metaphysics at the Stanford Encyclopedia of Philosophy.
The entry on properties at the Stanford Encyclopedia of Philosophy.
The entry on the Medieval problem of universals at the Stanford Encyclopedia of Philosophy.