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These terms are used to designate the theories that have been proposed as solutions of one of the most important questions in philosophy, often referred to as the problem of universals, which, while it was a favourite subject for discussion in ancient times, and especially in the Middle Ages, is still prominent in modern and contemporary philosophy. We propose to discuss in this article:

I. The Nature of the Problem and the Suggested Solutions;
II. The Principal Historic Forms of Nominalism, Realism, and Conceptualism;
III. The Claims of Moderate Realism.


The problem of universals is the problem of the correspondence of our intellectual concepts to things existing outside our intellect. Whereas external objects are determinate, individual, formally exclusive of all multiplicity, our concepts or mental representations offer us the realities independent of all particular determination; they are abstract and universal. The question, therefore, is to discover to what extent the concepts of the mind correspond to the things they represent; how the flower we conceive represents the flower existing in nature ; in a word, whether our ideas are faithful and have an objective reality.

Four solutions of the problem have been offered. It is necessary to describe them carefully, as writers do not always use the terms in the same sense.

A. Exaggerated Realism

Exaggerated Realism holds that there are universal concepts in the mind and universal things in nature. There is, therefore, a strict parallelism between the being in nature and the being in thought, since the external object is clothed with the same character of universality that we discover in the concept. This is a simple solution, but one that runs counter to the dictates of common sense.

B. Nominalism

Exaggerated Realism invents a world of reality corresponding exactly to the attributes of the world of thought. Nominalism, on the contrary, models the concept on the external object, which it holds to be individual and particular. Nominalism consequently denies the existence of abstract and universal concepts, and refuses to admit that the intellect has the power of engendering them. What are called general ideas are only names, mere verbal designations, serving as labels for a collection of things or a series of particular events. Hence the term Nominalism. Neither Exaggerated Realism nor Nominalism finds any difficulty in establishing a correspondence between the thing in thought and the thing existing in nature, since in different ways, they both postulate perfect harmony between the two. The real difficulty appears when we assign different attributes to the thing in nature and to the thing in thought; if we hold that the one is individual and the other universal. An antinomy then arises between the world of reality and world as represented in the mind, and we are led to inquire how the general notion of flower conceived by the mind is applicable to the particular and determinate flowers of nature.

C. Conceptualism

Conceptualism admits the existence within us of abstract and universal concepts (whence its name), but it holds that we do not know whether or not the mental objects have any foundation outside our minds or whether in nature the individual objects possess distributively and each by itself the realities which we conceive as realized in each of them. The concepts have an ideal value; they have no real value, or at least we do not know whether they have a real value.

D. Moderate Realism

Moderate Realism, finally, declares that there are universal concepts representing faithfully realities that are not universal.

How can there be harmony between the former and the latter? The latter are particular, but we have the power of representing them to ourselves abstractly. Now the abstract type, when the intellect considers it reflectively and contrasts it with the particular subjects in which it is realized or capable of being realized, is attributable indifferently to any and all of them. This applicability of the abstract type to the individuals is its universality. (Mercier, "Critériologie", Louvain, 1906, p. 343).


A. In Greek Philosophy

The conciliation of the one and the many, the changing and the permanent, was a favourite problem with the Greeks; it leads to the problem of universals. The typical affirmation of Exaggerated Realism, the most outspoken ever made, appears in Plato's philosophy ; the real must possess the attributes of necessity, universality, unity, and immutability which are found in our intellectual representations. And as the sensible world contains only the contingent, the particular, the unstable, it follows that the real exists outside and above the sensible world. Plato calls it eîdos, idea. The idea is absolutely stable and exists by itself (óntos ón; autá kath' autá), isolated from the phenomenal world, distinct from the Divine and human intellect. Following logically the directive principles of his Realism, Plato makes an idea entity correspond to each of our abstract representations. Not only natural species (man, horse) but artificial products (bed), not only substances (man) but properties (white, just), relations (double, triple), and even negations and nothingness have a corresponding idea in the suprasensible world. "What makes one and one two, is a participation of the dyad (dúas), and what makes one one is a participation of monad (mónas) in unity" (Phædo, lxix). The exaggerated Realism of Plato, investing the real being with the attributes of the being in thought, is the principal doctrine of his metaphysics.

Aristotle broke away from these exaggerated views of his master and formulated the main doctrines of Moderate Realism. The real is not, as Plato says, some vague entity of which the sensible world is only the shadow; it dwells in the midst of the sensible world. Individual substance (this man, that horse) alone has reality; it alone can exist. The universal is not a thing in itself; it is immanent in individuals and is multiplied in all the representatives of a class. As to the form of universality of our concepts (man, just), it is a product of our subjective consideration. The objects of our generic and specific representations can certainly be called substances (ousíai) , when they designate the fundamental reality (man) with the accidental determinations (just, big); but these are deúterai ousíai (second substances), and by that Aristotle means precisely that this attribute of universality which affects the substance as in thought does not belong to the substance (thing in itself); it is the outcome of our subjective elaboration. This theorem of Aristotle, which completes the metaphysics of Heraclitus (denial of permanent) by means of that of Parmenides (denial of change), is the antithesis of Platonism, and may be considered one of the finest pronouncements of Peripateticism. It was through this wise doctrine that the Stagyrite exercised his ascendency over all later thought.

After Aristotle Greek philosophy formulated a third answer to the problem of universals, Conceptualism. This solution appears in the teaching of the Stoics, which, as is known, ranks with Platonism and Aristoteleanism among the three original systems of the great philosophic age of the Greeks. Sensation is the principle of all knowledge, and thought is only a collective sensation. Zeno compared sensation to an open hand with the fingers separated; experience or multiple sensation to the open hand with the fingers bent; the general concept born of experience to the closed fist. Now, concepts, reduced to general sensations, have as their object, not the corporeal and external thing reached by the senses (túgchanon) , but the lektóon or the reality conceived; whether this has any real value we do not know. The Aristotelean School adopted Aristotelean Realism, but the neo-Platonists subscribed to the Platonic theory of ideas which they transformed into an emanationistic and monistic concepton of the universe.

B. In the Philosophy of the Middle Ages

For a long time it was thought that the problem of universals monopolized the attention of the philosophers of the Middle Ages, and that the dispute of the Nominalists and Realists absorbed all their energies. In reality that question, although prominent in the Middle Ages, was far from being the only one dealt with by these philosophers.

(1) From the commencement of the Middle Ages till the end of the 12th century.--It is impossible to classify the philosophers of the beginning of the Middle Ages exactly as Nominalists, Moderate and Exaggerated Realists, or Conceptualists. And the reason is that the problem of the Universals is very complex. It not merely involves the metaphysics of the individual and of the universal, but also raises important questions in ideology--questions about the genesis and validity of knowledge. But the earlier Scholastics, unskilled in such delicate matters, did not perceive these various aspects of the problem. It did not grow up spontaneously in the Middle Ages ; it was bequeathed in a text of porphyry's "Isagoge", a text that seemed simple and innocent, though somewhat obscure, but one which force of circumstances made the necessary starting-point of the earliest medieval speculations about the Universals.

Porphyry divides the problem into three parts:

  • Do genera and species exist in nature, or do they consist in mere products of the intellect ?
  • If they are things apart from the mind, are they coporeal or incorporeal things?
  • Do they exist outside the (individual) things of sense, or are they realized in the latter?
"Mox de generibus et speciebus illud quidem sive subsistant sive in nudis intelluctibus posita sint, sive subsistentia corporalia sint an incorporalia, et utrum separata a senaibilibus an in sensibilibus posita er circa haec subsistentia, decere recusabo." Historically, the first of those questions was discussed prior to the others: the latter could have arisen only in the event of denying an exclusively subjective character to universal realities. Now the first question was whether genera and species are objective realities or not: sive subsistant, sive in nudis intellectibus posita sint? In other words, the sole point in debate was the absolute reality of the universals : their truth, their relation to the understanding, was not in question. The text from Porphyry, apart from the solution he elsewhere proposed in works unknown to the early Scholastics, is an inadequate statement of the question; for it takes account only of the objective aspect and neglects the psychological standpoint which alone can give the key to the true solution. Moreover, Porphyry, after proposing his triple interrogation in the "Isagoge", refuses to offer an answer (dicere recusabo) . Boëthius, in his two commentaries, gives replies that are vague and scarecely consistent. In the second comentary, which is the more important one, he holds that genera and species are both subsistentia and intellecta (1st question), the similarity of things being the basis (subjectum) both of their individuality in nature and their universality in the mind : that genera and species are incorporeal not by nature but by abstraction (2nd question), and that they exist both inside and outside the things of sense (3rd question).

This was not sufficiently clear for beginners, though we can see in it the basis of the Aristotlean solution of the problem. The early Scholastics faced the problem as proposed by Porphyry: limiting the controversy to genera and species , and its solutions to the altenatives suggested by the first question: Do objects of concepts (i.e., genera and species ) exist in nature (subsistentia), or are they mere abstractions (nuda intelecta) ? Are they, or are they not, things? Those who replied in the affirmative got the name of Reals or Realists; the others that of Nominals or Nominalists. The former or the Realist, more numerous in the early Middle Ages (Fredugisus, Rémy d'Auxerre, and John Scotus Eriugena in the ninth century, Gerbert and Odo of Tournai in the Tenth, and William of Chapeaux in the twelfth) attribute to each species a universal essence (subsistentia) , to which all the subordinate individuals are tributary.

The Nominalists, who should be called rather the anti-Realists, assert on the contrary that the individual alone exists, and that the universals are not things realized in the universal state in nature, or subsistentia. And as they adopt the alternative of Porphyry, they conclude that the universals are nuda intellecta (that is, purely intellectual representations).

It may be that Roscelin of Compiègne did not go beyond these energetic protest against Realism, and that he is not a Nominalist in the exact sense we have attributed to the word above, for we have to depend on others for an expression of his views, as there is extant no text of his which would justify us in saying that he denied the intellect the power of forming general concepts, distinct in their nature from sensation. Indeed, it is difficult to comprehend how Nominalism could exist at all in the Middle Ages, as it is possible only in a sensist philosophy that denies all natural distinction between sensation and the intellectual concept. Futhermore there is little evidence of Sensism in the Middle Ages, and, as Sensism and Scholasticism, so also Nominalism and Scholasticism are mutually exclusive. The different anti-Realist system anterior to the thirteenth century are in fact only more or less imperfect forms of the Moderate Realism towards which efforts of the first period were tending, phases through which the same idea passed in its organic evolution. These stages are numerous, and several have been studied in recent monograph (e.g. the doctrine of Adélard of Bath, of Gauthier de Mortagne, Indifferentism, and the theory of the collectio ). The decisive stage is marked by Abélard, (1079-1142), who points out clearly the role abstraction, and how we represent to ourselves elements common to different things, capable of realization in an indefinite number of individuals of the same species, while the individual alone exists. From that to Moderate Realism there is but a step; it was sufficient to show that a real fundamentum allows us to attribute the general represention to individual thing. It is impossibe to say who was the first in the twelfth century to develop the theory in its entirety. Moderate Realism appears fully in the writing of John of Salisbury.

C. From the thirteenth Century

In the thirteenth century all the great Scholastics solved the problem of the universals by the theory of Moderate Realism ( Thomas Aquinas, Bonaventure, Duns Scotus ), and are thus in accord with Averroes and Avicenna, the great Arab commentators of Aristotle, whose works hasd recently passed into circulation by means of tranlations. St. Thomas formulates the doctrine of Moderate Realism in precise language, and for that reason alone we can give the name of Thomistic Realism to this doctrine (see below). With William of Occam and the Terminist School appear the strictly conceptualist solution of the problem. The abstract and universal concept is a sign (signum), also called a term ( terminus; hence the name Terminism given to the system), but it has no real value, for the absract and the universl do not exist in any way in nature and have no fundamentum outside the mind. The universal concept ( intentio secunda ) has as it object internal representations, formed by the understanding, to which nothing external corresponding can be attributed. The role of the universals is to serve as a label, to hold the place ( supponere ) in the mind of multitude of things which it can be attributed. Occam's Conceptualism would be frankly subjectivistic, if, together with the abstract concepts which reach the individual thing, as it exists in nature.

D. In Modern and Contemporary Philosophy

We find an unequivocal affirmation of Nominalism in Positivism. For Hume, Stuart Mill, Spencer, and Taine there is strictly speaking no universal concept. The notion, to which we lend universality, is only a collection of individual perceptions, a collective sensation, "un nom compris" (Taine), "a term in habitual association with many other particular ideas " (Hume), "un savoir potentiel emmagasiné" (Ribot). The problem of the correspondence of the concept to reality is thus at once solved, or rather it is suppressed and replaced by the psycological question: What is the origin of the illusion that induces us to attribute a distinct nature to the general concept, though the latter is only an elaborated sensation? Kant distinctly affirms the existence within us of abstract and general notions and the distinction between them and sensations, but these doctrines are joined with a characteristic Phonmenalism which constitutes the most original form of modern Conceptualism. Universal and necessary representations have no contact with external things, sinct they are produced exclusively by the structual functions (a priori forms) of our mind. Time and space, in which we frame all sensible impressions,cannot be obtained from expierence, which is individual and contigent; they are schemata which arise from our mental organization. Consequently, we have no warrant for establishing a real correspondence between the world of reality. Science, which is only an elaboration of the data of sense in accordance with other structural determinations of the mind (the categories), becomes a subjective poem, which has value only for us and not for a world outside us. A modern form of Platonic or Exaggerated Realism is found in the ontologist doctrine defended by certain Catholic philosophers in the middle of the nineteenth century, and which consist in identifying the objects of universal ideas with the Divine ideas or the archetypes on which the world was fashioned. As to Moderate Realism, it remains the doctrine of all those who have returned to Aristotleanism or adopted the neo-Scholastic philosophy .


This system reconciles the characteristics of external objects (particularity) with those of our intellectual representations (universality), and explains why science, though made up of abstract notions, is valid for the world of reality. To understand this it suffices to grasp the real meaning of abstraction. When the mind apprehends the essence of a thing (quod quid est; tò tí en eînai ), the external object is perceived without the particular notes which attach to it in nature (esse in singularibus), and it is not yet marked with the attribute of generality which reflection will bestow on it (esse in intellectu) . The abstract reality is apprehended with perfect indifference as regards both the individual state without and the universal state within: abstrahit ab utroque esse, secundum quam considerationem considerattur natura lapidis vel cujus cumque alterius, quantum ad ea tantum quæ per se competunt illi naturæ (St Tomas, "Quodlibeta", Q. i, a. 1). Now, what is thus conceived in the absolute state (absolute considerando) is nothing else than the reality incarnate in any give individual : in truth, the reality, represented in my concept of man, is in Socrates or in Plato. There is nothing in the abstract concept that is not applicable to every individual ; if the abstract concept is inadequate, because it does not contain the singular notes of each being, it is none the less faithful, or at least its abstract character does not prevent it from corresponding faithfully to the objects existing in nature. As to the universal form of the concept, a moment's consideration shows that it is subsequent to the abstraction and is the fruit of reflection: "ratio speciei accidit naturæ humanæ". Whence it follows that the universality of the concept as such is the work purely of the intellect : "unde intellectus est qui facit universalitatem in rebus" (St. Thomas, "De ente et essentia," iv).

Concerning Nominalism, Conceptualism, and Exaggerated Realism, a few general considerations must suffice. Nominalism, which is irreconcilable with a spiritualistic philosophy and for that very reason with scholasticism as well, presupposes the ideological theory that the abstract concept does not differ essentially from sensation, of which it is only a transformation. The Nominalism of Hume, Stuart Mill, Spencer, Huxley, and Taine is of no greater value than their ideology. The confound essentially distinct logical operations--the simple decomposition of sensible or empirical representations with abstraction properly so called and sensible analogy with the process of universalization. The Aristotleans recognize both of these mental operations, but they distinguish carefully between them. As to Kant, all the bounds that might connect the concept with the external world are destroyed in his Phenomenalism. Kant is unable to explain why one and the same sensible impression starts or sets in operation now this, now that category ; his a priori forms are unintelligible according to his own principles, since they are beyond experience. Moreover, he confuses real time and space, limited like the things they develop, with ideal or abstract time and space, which alone are general and without limit. For in truth we do not create wholesale the object of our knowledge, but we beget it within us under the causal influence of the object that reveals itself to us. Ontologism, which is akin to Platonic Realism, arbitrarily identifies the ideal types in our intellect, which come to us from the sensible world by means of abstraction, with the ideal types consubstantial with the essence of God. Now, when we form our first abstract ideas we do not yet know God. We are so ignorant of Him that we must employ these first ideas to prove a posteriori His existence. Ontologism has lived its life, and our age so enamoured of observation and experiment will scarcely return to the dreams of Plato.

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