Nominalism is the philosophical view that abstract concepts, general terms, or universals have no independent existence but exist only as names. It also claims that various individual objects labeled by the same term have nothing in common but their name. In this view, it is only actual physical particulars that can be said to be real, and universals exist only post res, that is, subsequent to particular things.
Nominalism is best understood in contrast to philosophical or ontological realism. Philosophical realism holds that when people use general terms such as "cat" or "green," those universals really exist in some sense of "exist," either independently of the world in an abstract realm (as was held by Plato, for instance, in his theory of forms) or as part of the real existence of individual things in some way (as in Aristotle's theory of hylomorphism). The Aristotelian type of realism is usually called moderate realism. As a still another alternative, there is a school called conceptualism, which holds that universals are just concepts in the mind. In the Middle Ages, there was a heated realist-nominalist controversy over universals.
History shows that after the Middle Ages, nominalism became more popularly accepted than realism. It is basically with the spirit of nominalism that empiricism, pragmatism, logical positivism, and other modern schools have been developed. But, this does not mean that any really satisfactory solution to the controversy has been found. So, even nominalism has developed more moderate versions such as "resemblance" nominalism and "trope" nominalism.
A careful observation shows that from among the various theories there seem to be two most promising ones: trope nominalism and moderate realism (especially Duns Scotus's moderate nominalism). They are most promising as genuine contenders because they both try to blur the traditional sharp distinction between universals and particulars. Any new promising solutions in the future, therefore, should probably blur this distinction in much the same way.
In the Middle Ages there was a controversy over universals. It arose from a passage in Boethius's translation of Porphyry's Isagoge sive quinque voces ("Introduction to Aristotle's Categories"), which raised the problem of genera and species: 1) as to whether they exist in nature or only in the mind; 2) whether, if they exist in nature, they are corporeal or incorporeal; and 3) whether they exist outside sensible particular things or are realized in them. Adherents to "realism" such as Bernard of Chartres (d. c. 1130), Saint Anselm (1033-1109), and William of Champeaux (1070-1121) held, like Plato, that universals alone have substantial reality, existing ante res (prior to particular things). Proponents of "nominalism" such as Berengar of Tours (c. 1010-1080) and Roscellinus (1050-1125), however, objected that universals are mere names, existing post res (subsequent to particular things) without any reality. The controversy was prominent in the late eleventh and twelfth centuries, and the issue was not only philosophical but also theological because it was quite evident that while realism represented a more spiritual type of worldview, nominalism showed a more anti-spiritual view. Realism, which recognized the substantial reality of universals separable from this world, was favorable to the theological teachings of the Church on God, heaven, soul, afterlife, etc. Realism was also favorable to the Church's other teachings such as the Trinity, the Eucharist, and original sin, which presupposed the substantial existence of universals. By contrast, nominalism turned out to be less favorable to the teachings of the Church. For example, the nominalist Roscellinus argued that "God" is no more than a name, and that the divine reality is only found in the three different individuals called Father, Son, and Holy Spirit. In 1092, Roscellinus was condemned for being a tritheist.
In order to mediate between realism and nominalism, Peter Abelard (1079-1142) suggested a position called "conceptualism." It rejects realism in favor of nominalism, when it says that universals have no substantial reality separable from the world of sensible things. However, it disagrees with nominalism, by maintaining that universals still exist as "concepts" in our minds, more than as mere names, thus being able to express real similarities in individual things themselves. But this position of conceptualism seems to be letting us come back to the same debate over the relationship of universals and individuals—albeit at a level—instead of answering it.
In the thirteenth century, great Scholastics such as Saint Thomas Aquinas (c. 1225-1274) and Duns Scotus (c. 1265-1308) dealt with the problem of universals from the viewpoint of what is usually called "moderate realism," largely under the influence of the philosophy of Aristotle that was reintroduced to the West through Islamic philosophy. Moderate realism locates universals in the mind like conceptualism and at the same time admits of their real basis in in rebus (in particular things). There was some difference, however, between Thomas's moderate realism and Duns Scotus's. For whereas the former was still saying that a universal of different individual things is numerically identical, the latter advanced the notion of a "common nature" (natura communis), a particularized universal, which is numerically different from one individual thing to another because it is made unique to a particular thing in which it exists, because of the "thisness" (haecceitas) of that individual thing. In any case, moderate realism was considered to be a best solution in the thirteenth century.
In the fourteenth century, however, nominalism was revived by the English Franciscan William of Ockham (c. 1285-1347), who had an impulse towards direct observation of the world rather than toward rational abstractions. He thus rejected any systematization of theology based on abstractions. He was interested in factual plurality in the world. So, he only accepted the Catholic theological teachings in the realm of faith. Gabriel Biel (c. 1420-1495), a follower of the nominalism of William of Ockham, theologically pursued this line of thought further, maintaining that theological dogma can properly be found in the realm of faith, not in the realm of reason. This perhaps helped to prepare the way for the Protestant Reformation in the sixteenth century.
The problem of universals arises from the question of how to account for the fact that some things are of the same type. For example, Fluffy and Kitzler are both cats, but what is this "catness" that both seem to have? Also, the grass, the shirt, and Kermit the Frog are green, but what is this quality of "green" that they all seem to have? There is the fact that certain properties are repeatable. Philosophers want to know in virtue of what are Fluffy and Kitzler both cats, and what makes the grass, the shirt, and Kermit green.
The answer of realism is that all the cats are cats in virtue of the existence of a universal, a single abstract thing, in this case, that is a part of all the cats. With respect to being cats, for Fluffy, Kitzler, and even the lion in the jungle, one of their parts is identical. In this respect, the three parts are literally one. "Catness" is repeatable because there is one thing that manifests itself, wherever there is a cat. This is the realism of Plato, who famously held that there is a realm of abstract forms or universals apart from the physical world, and that particular physical objects merely exemplify, instantiate, or "participate" in, the universals.
Nominalism denies the existence of universals in this sense of the term. The motivation to deny universals in this sense flows from several concerns. The first one is the question of where they exist. As Plato believed, are they located outside of space and time? Some assert that nothing is outside of space and time, though. In addition, what did Plato mean when he held that the several cows we see in the pasture, for example, all "participate" in the form of cow? What is "participation"? Didn't Plato, famously in his dialogue Parmenides, get tangled in confusion and unanswered questions, when he tried to specify just what or how a sensed thing (e.g., the individual cow) participates in a form (e.g., "cowness"). Plato also got into what seemed to him to be ethical and aesthetic problems, when he realized that the same arguments that would require that there be forms for noble things would also require that there be forms for ignoble things such as dirt or dung. To complicate things, what is the nature of the instantiation or exemplification of the logic of relation(s)? Also, when the realist maintains that all the instances of "catness" are held together by the exemplification relation, is this relation explained satisfactorily? Isn't it unusual that there could be a single thing (i.e., a form) that exists in multiple places simultaneously after being exemplified?
Moderate realists hold that there is no independent realm in which universals exist. They rather hold that universals are located in space and time, wherever they are manifest. Moderate realism can still recognize the laws of nature, based on the constants of human nature. Moderate realists of the twentieth century include Jacques Maritain and Etienne Gilson.
Today, however, some philosophers who delve into the workings of the human brain, such as Daniel Dennett, reject the idea that there is some "catness" in the real world. They believe that there are only circumstances that cause the brain to react with the judgment "cat." This nominalist tendency can also be seen amongst many philosophers who prefer simpler ontologies populated with only the bare minimum of types of entities, having "a taste for desert landscapes," to use the phrase of W.V. Quine. They attempt to express everything that they want to explain without using universals such as "catness" or "chairness."
Nominalism maintains that only individual things exist, arguing that the problem of universals can be handled only by properly thinking about individual things with respect to their natures and relations. Depending on how to think about individual things, there are various forms of nominalism ranging from extreme to almost-realist.
One extreme form is "pure" nominalism, or "predicate" nominalism, which maintains that Fluffy and Kitzler are both cats simply because the predicate "cat" linguistically applies to both of them. This form of nominalism is usually criticized of ignoring the problem of universals because it does not try to explain why that predicate correctly applies to Fluffy and Kitzler and not other predicates, simply leaving it a brute fact.
"Resemblance" nominalism believes 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 natural 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, however, 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 argue that it is not vicious.
Another form of nominalism is "trope" nominalism that attempts to build a theory of resemblance nominalism on a "theory of tropes." A trope (tropos in Greek, tropus in Latin, originally meaning "a turn") is a particular instance of a property or its name, far from a transcendent universal in the realist sense. It is the specific greenness of a shirt, for example. Therefore, it is numerically different from this green shirt to that green shirt, while being qualitatively identical beyond different green shirts. One might argue that there is a primitive objective resemblance relation that holds among tropes that are like each other. Others argue that all apparent tropes are constructed out of more primitive tropes and that the most primitive tropes are the entities of 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. According to D.M. Armstrong, a contemporary moderate realist, such a trope-based variant of nominalism has promise, although it may be unable to account for the laws of nature in the way his own theory of universals can.
Some modern Arabic philosophers have claimed in their studies of the history of Islamic philosophy that realist universals and the metaphysics related to the realist school of philosophy are incompatible with the Islamic worldview, and by trying to solve this problem they have developed the concept of nominalist universals.
Two exponents of nominalism in Medieval philosophy were the Islamic philosophers Ibn Khaldun and Ibn Taymiya.
History shows that nominalism was already widespread on the eve of the Protestant Reformation. The fact that Martin Luther studied at Erfurt under nominalist professors in his earlier years is sometimes considered to have contributed theologically to the rise of the Reformation. The spirit of nominalism was also there in the scientific movement of the Renaissance. Since then, nominalism became more and more accepted, giving rise to modern nominalistic traditions such as empiricism, pragmatism, instrumentalism, and logical positivism. Well-known nominalists include Francis Bacon, David Hume, John Stuart Mill, Herbert Spencer, and Nelson Goodman. Also, according to the philosopher of science Ian Hacking, much of what is called social constructionism of science in contemporary times is actually motivated by an unstated nominalist metaphysical view.
In spite of the widespread popularity of nominalism, however, the debate between realism and nominalism seems to be far from been settled. For in the twentieth century there still were many realists in the Catholic tradition such as German philosopher Dietrich von Hildebrand and British writer J.R.R. Tolkien. Also, there emerged influential moderate realists in the Neo-Thomist school such as Jacques Maritain and Etienne Gilson. Another well-known moderate realist is the Australian philosopher D.M. Armstrong. The question, therefore, is: Where can we find the solution to the debate?
Mary C. MacLeod and Eric M. Rubenstein admit of "our failure to find one [adequate solution]" to the problem of universals, but they present their useful observation that a consensus seems to be emerging that from among the diverse theories in the wide spectrum regarding universals, "two genuine contenders" are left: trope nominalism and moderate realism. It is understandable that trope nominalism is a "genuine contender," because when it posits "tropes" as particularized properties, which are not universals in the realist sense, it can explain the resemblance as well as difference of particular individual things through them. Trope nominalism looks promising, although it may still be a little too nominalist to be able to retain the laws of nature, according to D.B. Armstrong.
How about the other "genuine contender": moderate realism? As far as Duns Scotus' moderate realism is concerned, it ingeniously talks about universals in terms of "common natures" (naturae communis). According to this, common natures really exist in particular individual things, although when they exist in particular individual things, they are made individual by each particular individual thing's own principle of individuation, called "thisness" (haecceitas). This way, the difference as well as resemblance of individual things can be explained.
The "two genuine contenders" originally come from the two different traditions of nominalism and realism. So, "tropes" in trope nominalism are still within the nominalist tradition, while "common natures" in Scotus' moderate nominalism are broadly within the realist tradition. Interestingly, however, "tropes" and "common natures" are very similar because both of them are both universal and particular in character at once. This shows that any other promising solutions to the problem of universals in the future should probably blur the traditional sharp distinction between universals and particulars in much the same way.
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