Thomas Aquinas: Moderate Realism and the Universals


A. What the epistemological problem involves
B. Objectivity of external sensations
C. Real objectivity of abstract and general ideas – Universals
D. The Via Media between Naive Realism and Idealism
E. The nature of the mental synthesis
F. Conclusion


A. What the epistemological problem involves

It has been indicated that the epistemological problem centers upon an inquiry concerning the validity of our spontaneous assertions. This inquiry resolves itself into two problems. First, the motive which leads the mind to establish a relation between a subject and a predicate in a judgment, and secondly, the validity of the respective terms themselves. Thus, when I say that a number is odd or even, or, that water boils at 100 degrees C., I may inquire:

  • (a) What leads me to form a mental synthesis of number and odd or even; of water and boiling at 100 degrees C.?
  • (b) What is the validity of these terms: number; odd; even; water; boiling? Are they mere mental products or do they refer to objects independently existent in an external world?

Aquinas does not formulate these two problems with modern precision, for he wrote at a time when idealism and scepticism were mere academic theses which no one took seriously; but his doctrine contains a solution of the two problems which we have indicated.

We will begin with the second, and his answer may be summed up as follows: “Our sense perceptions correspond to an external world, but their content is not adequate or complete. Again our abstract and general ideas (water, life, number, equality, etc.) correspond to a reality which is not solely a product of the mind, since it has been inferred from sense data.”

B. Objectivity of external sensations

Generally speaking, according to the Schoolmen, the information presented to us by our senses is valuable, when working normally and when referring to their proper object, i.e., the special quality which each sense perceives to the exclusion of all the others (II, B). In the case then of color, sound, odors, quantitative state and shape of bodies, the sense data of sight, hearing, smell, touch, were considered as infallible. “The senses announce to us as they are themselves affected or modified.” Nuntiant uti afficiuntur [1].

Do our senses give us not only accurate information concerning the material world, but also adequate knowledge? Scholasticism is prevented from admitting this in virtue of its basic principles, since in every act of cognition we contribute something of our own. Color cannot exist in my visual organ in the same way that it exists outside. But the problem of the extent to which our sensations correspond to the external world was neglected in the thirteenth century. The illusions of the senses were indeed known at that time; but as will be seen it was held that the erroneous information which resulted therefrom was not imputable to the senses as such. At the most they conceded to the perceptions of touch the privilege of giving us the most intimate contact of all with reality, since continuous quantity, which is perceived by the sense of touch, is the fundamental attribute of material things, resulting from its very nature [2]. The Schoolmen were not aware of the distinction between primary and secondary qualities, in the sense introduced by Descartes and Locke. They held that quantity and extension do not constitute the essence of bodies (as Descartes thought), but rather its fundamental property.

C. Real objectivity of abstract and general ideas – Universals

An abstract idea has the same validity as a sensation, for it is from the content of sensation that the content of our ideas is derived. This content — including that of the highest and most general concepts, such as cause, life, substance — is contained in some way in the complexus of reality grasped by our senses; for, obviously, if they were not somehow in sense data, they could never have been derived from it.

But, there is a special difficulty when we come to consider what sort of correspondence can exist between reality and the concepts, each of which represent some aspect of it. We cam across the same difficulty previously, when dealing with the origin of ideas (III, B). Here the difficulty concerns their validity. Outside us, everything is individual; the universe of the Schooolmen is a pluralistic universe, composed of single substances (VIII, A), and everything which affects these individual substances is particularized. This being so, how can there be any correspondence between that which is concrete and singular (e.g., this living being, this material movement) on the one hand, and the abstract, universal notion (life, motion) on the other? Such is the famous problem of Universals, — or rather of the validity of our abstract and universal ideas.

Aquinas replies that the correspondence “between ideas and individual realities is not adequate, but is none the less faithful.” To prove this, let us distinguish, as he does, between the abstract character of the idea, and its universality.

Consider the character of abstractions, which is the primordial one. We already know that the content of the concept ‘man,’ ‘life,’ ‘local motion’ is considered apart from those particular characteristics inseparable from each individual man, or each living being, or instance of local motion. As viewed by the mind, reality is neither one nor multiple; it seems to be completely indifferent to anything connected with number. The concept simply expresses the whatness of the reality ‘man,’ ‘movement,’ ‘life.’ In consequence, the abstract concept is a faithful representation of reality, for all the elements which go to make up the whatness or essence of ‘man,’ or ‘life’ or ‘motion’ are found in each individual man or movement. Abstraction does not falsify (abstrahentium non est mendacium).

But the concept, although faithful to, is not entirely commensurate with concrete things, for the mind neglects the hallmark of individuality which differentiates each particular man, living being or movement from others, and is incapable of knowing it. The abstract concept teaches us nothing concerning the essence of the individual. Moreover, not only is it true that the hallmark of individuality escapes the mind, but our idea of a living beings does not take account of the differences in essence between living beings of several kinds. The more abstract our knowledge is, the less it conveys of reality. The human mind has nothing to be proud of. Feeble and weak, but reliable in the little that they teach us, — such is the nature of our abstract ideas.

As for the process of universalization, which the abstract idea undergoes, this is entirely the work of the mind, for it consists in attributing to the content of the abstract idea an indefinite elasticity, and enables us to realize for instance that the essence of local motion or of humanity is found identically and completely in all instances of local motion, and in all human beings, whether actually existing or only possible. The characteristic of universality is the result of a reflection. Peter or John do not admit of multiplication. Universals do not exist outside of us; they exist only in our understanding. On the other hand, the whatness to which our mind gives the form of universality has a foundation in the extra-mental world. The process of universalizing neither takes away nor adds anything to the validity of the abstract ideas. Universale est formaliter in intellectu, fundamentaliter in rebus. Such is the condensed formula which sums up the thomistic solution of the problem. It was not discovered by Aquinas, but is rather the result of a slow and painful elaboration by Western thought in general. We find already in Abaelard, who flourished in the twelfth century, this doctrine of sound common sense, which fits in so well with the individualism of the Feudal system.

D. The Via Media between Naive Realism and Idealism

The thomistic doctrine of the correspondence between sense perceptions and abstract ideas on the one hand, and the external world on the other hand may be called the via media between naive realism and idealism.

For the person whom we call a ‘naive realist,’ reality is altogether independent of our knowledge of it, and our minds faithfully and accurately reflect things just as they are outside of us, in a merely passive way. The external world is reflected in consciousness as in a mirror. Scholasticism rejects this explanation of the absolute correspondence between the world of reality and the world of thought, as being too superficial, and instead gives us the conception of knowledge as a complex phenomenon, the product of two factors, — the object known and the subject knowing. The knower invests the thing known with something of himself.

Does this imply that the known object is simply a product of our mental organization, and that we know directly only our internal or subjective modifications? This doctrine, which is that of idealism, is equally opposed to the scholastic conception. For, according to the latter, the real object plays a part in knowledge, and is present to us in the act of knowing. We directly attain to reality and being, — so much so that the process by which reality acts upon us, the impression received, is discovered only as the result of reasoning (III, A).

The epistemology of Aquinas is thus a moderate realism, a via media between exaggerated or naive realism, and idealism. We attain to a reality itself independent of our act of knowing, and in doing so we become possessed of knowledge which is true, but inadequate. The process of psychological elaboration which goes on in the mind limits the field of knowledge, but does not disfigure it.

E. The nature of the mental synthesis

The second problem, which we must examine now, is to find out whether we have a plausible motive for joining two ideas in a judgment, and what is that motive. We may reply with Thomas: “The motive for the mental synthesis is the very nature of the represented objects.” It is the nature of what we call water, ebullition; number, even, odd, which leads the mind to unite them, in the first case with, in the second case without the aid of experience.

This correspondence between represented objects constitutes truth. As soon as the connection between the content of the subject and that of the predicate appears to the mind, in other words becomes evident to it, the mind asserts it; and certainty is nothing but the firm adhesion of the mind to what it perceives.

It is important to note that the mind merely perceives the connection, without creating it, and herein lies the difference between thomistic and kantian intellectualism.

This doctrine applies to all judgments, and therefore to those directing principles which we have called the laws of universal intelligibility. For instance, in the principle of contradiction, the motive of our assertion is our insight into the incompatibility of being and non-being. The question of applicability of these principles to existing beings follows immediately, once the existence of such extra-mental reality has been proved. Given that being exists, no matter of what kind, I have the right to declare it incompatible with non-being. Now if there is such a thing as contingent being, I am justified in applying to it that which belongs to the inmost nature of all contingent beings (3).

Another corollary of this doctrine is that error is a property of judgment only. Error can belong neither to existing beings, nor to sensations, nor to simple apprehensions. Thomas employs this theory to solve the problem of sense illusions. The senses affirm nothing: they do not reflect upon the data, but present them just as they are, without any interpretation. That which is sweet to the palate of a healthy man appears bitter to an invalid (4). Consequently the senses can neither correct themselves, nor find out the causes of their failures or illusions. Reason must intervene to test and control, and separate the true from the false. Error comes in with the judgment, for instance, when we rely on our sense-perception in predicating an attribute which the sensation in question is not competent to give (II, B); or else a content which is disfigured because of the abnormal condition of the organism. In any case, we possess means of controlling the illusions of the senses, and an illusion which is capable of control is no longer really deceptive.

F. Conclusion

We perceive directly reality itself, and not our subjective modification of it. We perceive it thanks to a close collaboration between sense and intellect. The abstractive work of the mind, either superficial or profound, accompanies all our sense knowledge, and the mind has a tendency to unify all the data, and to arrive at an intelligible object that is increasingly complete. The mind is ever on the lookout for being, and seizes it whenever it presents itself. Intellectus potest quodammodo omnia fieri. — “The mind can in a way become all things.” But it grasps reality imperfectly. The reflective study of the epistemological problem throws light upon the spontaneous operation of the mind.

Reflection makes it evident that truth is found only in a judgment. Secundum hoc cognoscit veritatem intellectus quod supra se ipsum reflectitur. — The mind knows truth inasmuch as it reflects back upon itself. It also makes it evident that mind in its spontaneous judgments seizes reality. Therefore Thomas is led to add that mind is made naturally to attain reality, in conjus natura est ut rebus conformatur (5).

Taking what precedes into consideration, we may summarize thomistic doctrine in that well-known formula, current in the thirteenth century: truth is the correspondence between reality and the mind, veritas est adaequatio rei et intellectus.



  1. Summa Theol., Ia, q. 17, art. 2, or again: “Non decipitur (sensus) circa objectum proprium.” The senses do not err concerning their proper object.

  2. Sensus tactus quansi fundamentum aliorum sensuum. De Veritate, q. 22, art. 5. It is possible to give a direct proof of the objectivity of external sensations by means of the principle of causality. A sensation is a non-necessary or a contingent event; it might not have taken place. In consequence, it has not within itself a sufficient explanation of its existence, — it depends upon something else (IV, 2). This ‘other’ is not-myself, for consciousness bears witness that I am passive in sensation. We accordingly conclude that this other is different from myself, and that there exists a real non-ego, which is the cause of the vital excitation culminating in the act of sensation. By elimination, it can be proved that this non-ego is none other than the material world. This reasoning, which we do not meet in the texts of Thomas, is quite in the spirit of his philosophy.

  3. Certainly the principles of which we speak are independent of experience in the sense that the bond of union between the subject and predicate does not depend upon the existence of the material universe (III, B), but if this world exists — and it does exist — then the principles of being must govern it.

  4. Summa Theol., Ia, q. 17, art. 2. De Veritate, q. 1, art. 10.

  5. De Veritate, q. 1, art. 9.