Realism

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Nominalism is the position in metaphysics that there exist no universals outside of the mind.

Nominalism is best understood in contrast to realism. Philosophical realism holds that when we use descriptive terms such as "green" or "tree," the Forms of those concepts really exist, independently of world in an abstract realm. Such thought is associated with Plato. Nominalism, by contrast, holds that ideas represented by words have no real existence beyond our imaginations.

The problem of universals
Nominalism arose in reaction to the problem of universals. Specifically, accounting for the fact that some things are of the same type. For example, Fluffy and Kitzler are both cats, or, the fact that certain properties are repeatable, such as: the grass, the shirt, and Kermit the Frog are green. One wants to know in virtue of what makes Fluffy and Kitzler both cats and what makes the grass, the shirt, and Kermit green.

The realist answer is that all the green things are green in virtue of the existence of a universal; a single abstract thing, in this case, that is a part of all the green things. With respect to the colour of the grass, the shirt and Kermit, one of their parts is identical. In this respect, the three parts are literally one. Greenness is repeatable because there is one thing that manifests itself wherever there are green things.

Nominalism denies the existence of universals. The motivation to deny universals flows from several concerns. The first one concerns where they exist. Plato famously held that there is a realm of abstract forms or universals apart from the physical world. Particular physical objects merely exemplify or instantiate the universal. But this raises the question: Where is this universal realm? One possibilty is that it is outside of space and time. However, some assert that nothing is outside of space and time. To complicate things, what is the nature of the instantiation or exemplification relation?

Moderate realists hold that there is no realm in which universals exist, but rather universals are located in space and time wherever they are manifest. Now, recall that a universal, like greenness, is supposed to be a single thing. Nominalists consider it unusual that there could be a single thing that exists in multiple places simultaneously. The realist maintains that all the instances of greenness are held together by the exemplification relation, but this relation cannot be explained.

Finally, many philosophers prefer simpler ontologies populated with only the bare minimum of types of entities, or as W. V. Quine said "They have a taste for 'desert landscapes'". They attempt to express everything that they want to explain without using universals such as "catness" or "chairness".

Nominalism in the Islamic Philosophy
Some of modern Arabic pilosophers has claimed in their studies about the History of Islamic philosophy that realist universals and the Metaphysics related to this Realism school of Philosophy has formed problem to be compatible with the Islamic worldview, and through trying to solve this problem they develop the concept of nominalist universal.

The frank Expression for Nominalism in Medival Philosophy has been made by two of the late-aged Islamic philosophers, Ibn Khaldoun and Ibn Taymiya.

Varieties of nominalism
There are various forms of nominalism ranging from extreme to almost-realist. One extreme is "predicate" nominalism. Fluffy and Kitzler are both cats simply because the predicate 'cat' applies to both of them. However, the realist will object as to what the predicate applies to.

Resemblance nominalists believe that 'cat' applies to both cats because Fluffy and Kitzler resemble an exemplar cat closely enough to be classed together with it as members of its kind, or that they differ from each other (and other cats) quite less than they differ from other things, and this warrants classing them together. Some resemblance nominalists will concede that the resemblance relation is itself a universal, but is the only universal necessary. This betrays the spirit of nominalism. Others argue that each resemblance relation is a particular, and is a resemblance relation simply in virtue of its resemblance to other resemblance relations. This generates an infinite regress, but many agree that it is not vicious.

Another form of nominalist is one that attempts to build a theory of resemblance nominalism on a theory of tropes. A trope is a particular instance of a property, like the specific greenness of a shirt. One might argue that there is a primitive, objective resemblance relation that holds among like tropes. Another route is to argue that all apparent tropes are constructed out of more primitive tropes and that the most primitive tropes are the entities of complete physics. Primitive trope resemblance may thus be accounted for in terms of causal indiscernibility. Two tropes are exactly resembling if substituting one for the other would make no difference to the events in which they are taking part. Varying degrees of resemblance at the macro level can be explained by varying degrees of resemblance at the micro level, and micro-level resemblance is explained in terms of something no less robustly physical than causal power. Armstrong, perhaps the most prominent contemporary realist, argues that such a trope-based variant of nominalism has promise, but holds that it is unable to account for the laws of nature in the way his theory of universals can.

Ian Hacking has also argued that much of what is called social constructionism of science in contemporary times is actually motivated by an unstated nominalist metaphysical view. For this reason, he claims, scientists and constructionists tend to "shout past each other."

Strong proponents of this school of thought include John Locke and George Berkeley.