Universals

Universals or properties are posited by philosophers to explain the similarity of numerically different things. When we say, for instance, that there are "two people driving the same car", it is common sense for most people that they are driving two different cars but those cars have shared properties - they are made by the same company, are the same color, have the same engine and so on. In the language of this branch of metaphysics, the two cars are particulars, but those particular cars share properties, attributes or universals.

In discussion of universals, properties are usually distinguished from relations: a person may drive a red car, but they may be three inches taller than their brother. The former is a property, but the latter is a relation.

How do philosophers understand universals? Quite simply, the two main opinions held are those of realism and nominalism. Realists hold that universals exist in some sense - that a red bus and a red book both have the property of red, and that this property is the same in both. Nominalists, on the other hand, deny this - stating instead that there are similarities between things, but these are not anything more than 'tropes'.

The first philosopher to discuss properties that we know of is Plato in the Parmenides dialogue, and throughout the other dialogues as 'the Forms'. One problem with the notion of universals is whether it is possible for a universal to exist even if no particulars hold that universal. According to Plato, the Forms have a non-physical existence - that even if we were to destroy all the bottles in the world, the Form of the bottle would exist both before and after the particular individual bottles. Aristotle rejected the idea of universals existing outside of the particular instances. The Form of a bottle exists inside each individual bottle. Realism has been held by modern philosophers including Bertrand Russell and D. M. Armstrong, while nominalism's twentieth-century advocates have included Willard Van Orman Quine and David Lewis. Trope theory is another response to the problem of universals, and is advocated by Chris Daly, D. C. Williams, G. F. Stout and J. Cook Wilson - the former two are nominalist trope theorists while the latter is a realist.

Forms of nominalism

There are a variety of ways to try and explain sameness of type without admitting a distinction between universals and particulars - these are forms of nominalism. Nominalists tend to say that we quite naturally talk about properties and relations between things, but we cannot infer the existence of universals from mere talk (any more than we could infer the existence of God or natural rights from the utterances of 'Thank God!' or 'You are breaching my rights.'). Why deny the existence of universals beyond the mere talk of universals? A fundamental desire of those in metaphysics is to follow Ockham's razor and reduce the number of things in existence to the minimum number we absolutely need to posit.

A simple form of nominalism is that of Predicate Nominalism. A Predicate Nominalist says that something is, say, green because the predicate 'is green' can be applied to it. D. M. Armstrong provides some good reasons for rejecting predicate nominalism. Predicate nominalists have transferred the explanation of a thing's greenness from the thing to the word that represents the property. This has only shifted the problem. What is it that makes it so that 'grass is green' is true and 'the stapler is green' is not? It also leads to some counter-intuitive results - few sensible people would deny that an atom is made up of protons, neutrons and electrons - and that the predicate 'contains electrons' has a truth value. They would agree that things have always been made up of protons, electrons and neutrons, even before we had any words to describe them. Even if no predicate existed to describe how the atom contains electrons, the property is still there. Armstrong states that the Predicate Nominalist has to give an account of this phenomena using the language of "possible predicates". Realism and other forms of nominalism dodge this difficulty.

Class Nominalism is the view that for a thing to be of a certain type is to say that it exists within a particular class (strictly, in a particular set). Armstrong gives a very simple negation of this idea by pointing to empty classes. The class of all unicorns is empty and the class of, say, goblins is also empty. The nil class - that is, the class with no members - is the same. Thus, being a unicorn ultimately becomes the same as being a goblin. This seems counterintuitive for most people. Even if they grant that unicorns do not exist and goblins do not exist, if something were to appear in front of them, they could be reasonably sure of whether or not it is a unicorn. Even if a list of necessary and sufficient conditions could not be drawn up of what makes something a unicorn, enough conditions could be listed to state that something cannot be both a unicorn and a goblin.

Class nominalist theories have to figure out a way of distinguishing those classes which are natural from other classes. The class of all red things is more natural in some sense than a all things which are either red or older than fifty years old. Anthony Quinton suggests that this notion of naturalness is primitive and not analyzable any further. Against Quinton, resemblance nominalists like H. H. Price suggests that we can determine natural classes on the basis how the things inside that class resemble each other.

Some nominalists have just refused to provide any expansion of their theories to explain some of the problems that realists see in them. Michael Devitt has coined the rather amusing term 'ostrich nominalist' to describe those who adopt a nominalist position whilst ignoring the One over Many argument (he also refers to those who accept realism solely on the basis of the One over Many argument to be 'mirage realists'