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(Latin idea, forma, species; Greek idea , eidos , from idein , to see; French idée; German Bild; Begriff )

Probably to no other philosophical term have there been attached so many different shades of meaning as to the word idea. Yet what this word signifies is of much importance. Its sense in the minds of some philosophers is the key to their entire system. But from Descartes onwards usage has become confused and inconstant. Locke, in particular, ruined the term altogether in English philosophical literature, where it has ceased to possess any recognized definite meaning. He tells us himself at the beginning of his "Essay on the Human Understanding" that in this treatise "the word Idea stands for whatever is the object of the understanding when a man thinks. I have used it to express whatever is meant by phantasm, notion, species, or whatever it is which the mind can be employed about when thinking." In fact, with him it denotes, indifferently, a sensation, a perception, an image of the imagination, a concept of the intellect, an emotional feeling, and sometimes the external material object which is perceived or imagined.


The word was originally Greek, but passed without change into Latin. It seems first to have meant form, shape, or appearance, whence, by an easy transition, it acquired the connotation of nature, or kind. It was equivalent to eidos , of which it is merely the feminine, but Plato's partiality for this form of the term and its adoption by the Stoics secured its ultimate triumph over the masculine. Indeed it was Plato who won for the term idea the prominent position in the history of philosophy that it retained for so many centuries. With him the word idea , contrary to the modern acceptance, meant something that was primarily and emphatically objective, something outside of our minds. It is the universal archetypal essence in which all the individuals coming under a universal concept participate. By sensuous perception we obtain, according to Plato, an imperfect knowledge of individual objects; by our general concepts, or notions, we reach a higher knowledge of the idea of these objects. But what is the character of the idea itself? What is its relation to the individual object? And what is its relation to the author or originator of the individual things? The Platonic doctrine of ideas is very involved and obscure. Moreover, the difficulty is further complicated by the facts that the account of the idea given by Plato in different works is not the same, that the chronological order of his writings is not certain, and, finally, still more because we do not know how far the mythological setting is to be taken literally. Approximately, however, Plato's view seems to come to this: — To the universal notions, or concepts, which constitute science, or general knowledge as it is in our mind, there correspond ideas outside of our mind. These ideas are truly universal. They possess objective reality in themselves. They are not something indwelling in the individual things, as, for instance, form in matter, or the essence which determines the nature of an object. Each universal idea has its own separate and independent existence apart from the individual object related to it. It seems to dwell in some sort of celestial universe ( en ouranio topo ). In contrast with the individual objects of sense experience, which undergo constant change and flux, the ideas are perfect, eternal, and immutable. Still, there must be some sort of community between the individual object and the corresponding idea, between Socrates and the idea "man", between this act of justice and the idea "justice". This community consists in "participation" ( methexis ). The concrete individual participates, or shares, in the universal idea, and this participation constitutes it an individual of a certain kind or nature. But what, then, is this participation, if the idea dwells in another sphere of existence ? It seems to consist in imitation ( mimesis ). The ideas are models and prototypes, the sensible objects are copies, though very imperfect, of these models. The ideas are reflected in a feeble and obscure way in them. The idea is the archetype ( paradeigma ), individual objects are merely images ( eidola ). Finally, what precisely is the celestial universe in which the ideas have eternally existed, and what is their exact relation to God or to the idea of the good? For Plato allots to this latter a unique position in the transcendental region of ideas. Here we meet a fundamental difference between the answers from two schools of interpreters.


Aristotle, who, his critics notwithstanding, was as competent as they to understand Plato, and was Plato's own pupil, teaches that his master ascribed to the various ideas an independent, autonomous existence. They are a multiplicity of isolated essences existing separated from the individual objects which copy them, and they are united by no common bond. All the relations subsisting in the hierarchies of our universal concepts, however, seem in Plato's view to be represented by analogous relations amongst the autonomous ideas. Aristotle's interpretation was accepted by St. Thomas and the main body of the later Scholastics ; and much pain has been devoted to establishing the absurdity of this alleged theory of separation. But the ultra realism of the Platonic theory of ideas was susceptible of a more benevolent interpretation, which, moreover, was adopted by nearly all the early Fathers of the Church. Indeed they found it easier to Christianize his philosophy than did Albertus Magnus and St. Thomas to do the like for that of Aristotle. They unanimously understood Plato to locate this world of ideas in the Mind of God, and they explained his kosmos nontos as a system of Divine conceptions — the archetypes according to which God was to form in the future the various species of created beings. With respect to the origin of our knowledge of these universal ideas, Plato cannot consistently derive it from sensuous experience. He therefore teaches that our universal concepts, which correspond to these ideas, are, strictly speaking, innate, inherited by the soul from a previous state of existence. There, in that transcendental Eden, the soul, by direct contemplation of the ideas, acquired these concepts. Sensible experience of the objects around us now merely occasions the reminiscence of these pre-natal cognitions. The acquisition of knowledge is thus, strictly speaking, a process of recollection. Aristotle vigorously attacked Plato's theory of universal ideas. He himself teaches that sensible experience of the concrete individual is the beginning and foundation of all cognition. Intellectual knowledge, however, is concerned with the universal. But it must have been derived from the experience of the individual which, therefore, in some way contains the universal. The universal cannot exist, as such, apart from the individual. It is immanent in the individual as the essence, or nature, specifically common to all members of the class. Since this essence, or nature, constitutes the thing specifically what it is, man, horse, triangle, etc., it furnishes the answer to the question: What is the thing? ( Quid est? ). It has therefore been termed the quiddity of the thing. In Greek, according to Aristotle, the to ti en enai, eidos, morphe , and ousia deutera are one and the same thing — the essence, or quiddity, which determines the specific nature of the thing. This is the foundation for the general concept in the mind, which abstracts the universal form ( eidos nonton ) from the individual. Several of the early Fathers, as we have said, interpreted Plato benevolently, and sought to harmonize as much of his doctrine as possible with Christian theology. For them the ideas are the creative thoughts of God, the archetypes, or patterns, or forms in the mind of the Author of the universe according to which he has made the various species of creatures. "Ideæ principales formæ quædam vel rationes rerum stabiles atque incommutabiles, quæ in divinâ intelligentiâ continentur" (St. August., "De Div.", Q. xlvi). These Divine ideas must not be looked on as distinct entities, for this would be inconsistent with the Divine simplicity. They are identical with the Divine Essence contemplated by the Divine Intellect as susceptible of imitation ad extra .

Scholastic Period

This doctrine of the Fathers received its complete elaboration from the Schoolmen in the great controversy concerning universals ( de universalibus ) which occupied a prominent place in the history of philosophy from the tenth to the thirteenth century. The ultra-realists tended towards the Platonic view in regard to the real existence of universal forms, as such, outside of the human mind, though they differed as to their explanation of the nature of this universality, and its participation by the individuals. Thus William of Champeaux seems to have understood the universal to exist essentially in its completeness in each individual of the species. In essence these individuals are but one, and whatever difference they have is one of accidents, not of substance. This would lead to a pantheistic conception of the universe, akin to that of Scotus Eriugena. On the other hand, the extreme Nominalist view, advocated by Roscelin, denies all real universality, except that of words. — A common name may be applied to the several objects of a species or genus, but neither in the existing individuals nor in the mind is there a genuine basis or correlate for this community of predication. The Aristotelean doctrine of moderate realism, which was already in possession before the eleventh century, held its ground throughout the whole period of Scholasticism, notwithstanding the appearance of distinguished champions of the rival hypothesis, and at last permanently triumphed with the establishment of the authority of St. Thomas. This theory, which in its complete form we may call the Scholastic doctrine of universals, distinguished universalia ante res, in rebus, et post res . The universal exists in the Divine Mind only as an idea, model, or prototype of a plurality of creatures before the individual is realized. Genus or species cannot in order of time precede the individual. Plato's separate ideas, did they physically exist, would have been individualized by their existence and have thus ceased to be universals. The universal exists in the individual only potentially or fundamentally, not actually or formally as universal. That is, in each of the individuals of the same species there is a similar nature which the mind, exercising its abstractive activity, can represent by a concept or idea as separate, or apart, from its individualizing notes. The nature, or essence, so conceived is capable of being realized in an indefinite number of individuals, and therefore was justly described as "potentially universal". Finally, by a subsequent reflective generalizing act, the mind considers this concept, or idea, as representative of a plurality of such individuals, and thereby constitutes it a formally universal concept, or idea. In fact, it is only in the concept, or idea, that true universality is possible, for only in the vital mental act is there really reference of the one to the many. Even a common name, or any other general symbol, viewed as an entity, is merely an individual. It is its meaning, or significant reference, that gives it universality. But the fact that in the external world individual beings of the same species, e.g., men, oak trees, gold, iron, etc., have perfectly similar natures, affords an objective foundation for our subjective universal ideas and thereby makes physical science possible.

Diverse Meaning of Idea with Medieval and Modern Scholastic Writers

We have just been using the term idea in its modern Scholastic sense as synonymous with "concept". By the Schoolmen the terms conceptio, conceptus mentis, species intelligibilis, and verbum mentale were all employed, sometimes as equivalents and sometimes as connoting slight differences, to signify the universal intellectual concepts of the mind. The term idea, however, probably in consequence of the Platonic usage, was for a long period employed chiefly, if not solely, to signify the forms or archetypes of things existing in the Divine Mind. Even when referred to the human mind, it commonly bore the significance of forma exemplaris , the model pictured by the practical intellect with a view to artistic production, rather than that of a representation effected in the intellect by the object apprehended. The former was described as an exercising of the "practical", the latter of the "speculative", intellect, though the faculty was recognized as really the same. St. Thomas, however, says that idea may stand for the act of the speculative intellect also — "Sed tamen si ideam communiter appellamus similitudinem vel rationem, sic idea etiam ad speculativam cognitionem pure pertinere potest" (QQ. Disp. de Ideis, a. 3). But I have not been able to find any passage in which he himself employs the word idea in the modern Scholastic sense, as equivalent to the intellectual concept of the human mind. The same is true as regards Francisco Suárez; so that the recognized general usage of the term in modern Scholastic textbooks does not seem to go much farther back than the time of Descartes.

Modern Philosophy

Passing from the Schoolmen to modern philosophy, whilst, among those Catholic writers who adhered in general to the medieval philosophy, the term idea came to be more and more used to designate the intellectual concept of the human mind, outside of the Scholastic tradition it was no longer confined to intellectual acts. Descartes seems to have been the first influential thinker to introduce the vague and inaccurate use of the word idea which characterizes modern speculation generally. Locke, however, as we have mentioned, is largely responsible for the confusion in respect to the term which has prevailed in English philosophical literature. Descartes tells us that he designates generally by the term idea "all that is in our minds when we conceive a thing"; and he says, in another place, "idea est ipsa res cogitata quatenus est objective in intellectu." The Cartesian meaning of idea seems, then, to be the general psychical determinant of cognition. This wide signification was generally adopted by Gassendi, Hobbes, and many other writers, and the problem of the origin of ideas became that of the origin of all knowledge. There is, however, throughout, a reversal of the Platonic usage, for in its modern sense idea connotes something essentially subjective and intra-mental. With Plato, on the other hand, the ideas were emphatically objective. Spinoza defined idea as mentis conceptus , and warned his readers to distinguish it from phantasms of the imagination, imagines rerum quas imaginamus . We have cited at the beginning of this article Locke's vague definition. The confused and inconsistent usage to which he gave currency contributed much to the success of Berkeley's idealism and Hume's scepticism. From the position frequently adopted by Locke, that ideas are the object of our knowledge, that is, that what the mind knows or perceives are ideas, the conclusions drawn by Berkeley, that we have therefore no justification for asserting the existence of anything else but ideas, and that the hypothesis of a material world, the unperceived external causes of these ideas, is useless and unwarranted, was an obvious inference. Hume starts with the assumption that all cognitive acts of the mind may be divided into "impressions" (acts of perception), and "ideas", faint images of the former, and then lays down the doctrine that "the difference between these consists in the degrees of force and liveliness with which they strike on the mind." He then shows without much difficulty that genuine knowledge of reality of any kind is logically impossible. Kant assigned quite a new meaning to the term. He defines ideas as "concepts of the unconditioned which is thought of as a last condition for every conditioned". The transcendental ideas of metaphysics with him are, God, freedom, and immortality, "a pure concept" ( ein reiner Begriff ) may be either a Verstandesbegriff (notion), or a Vernunftbegriff (idea), the difference being that "the latter transcends the possibility of experience." In the Hegelian philosophy the term again assumed an objective meaning, though not that of Plato. It is a name for the Absolute and the World process viewed as a logical category. It is the absolute truth of which everything that exists is the expression.

Such being the varying signification of the term in the history of philosophy, we may now return to consider more closely its adopted meaning among Catholic philosophers. The term idea , and especially universal idea , being generally accepted by them as equivalent to universal concept, it is the product of the intellect, or understanding, as distinguished from the sensuous faculties. It is an act of the mind which corresponds to a general term in ordinary speech. Thus, in the sentence, "water is composed of oxygen and hydrogen", the three words water , oxygen , and hydrogen stand for any genuine samples of these substances. The names have a definite yet universal meaning. The mental act by which that universal meaning is realized is the universal idea. It is a quite distinct thing from the particular sensation or image of the imagination, more or less vivid, which may accompany the intellectual act. The image may be distinct or confused, lively or feeble. It probably varies from moment to moment. It is felt to be of a subjective, contingent, and accidental character, differing considerably from the corresponding image in other persons' minds. It is, however, always an individualistic concrete entity, referring to a single object. Not so, however, with the intellectual idea. This possesses stability. It is unchangeable, and it is universal. It refers with equal truth to every possible specimen of the class. Herein lies the difference between thought and sensuous feeling, between spiritual and organic activity (see INTELLECT).


Given the fact that the human mind in mature life is in possession of such universal ideas, or concepts, the question arises: How have they been attained? Plato, as we have incidentally observed, conceives them to be an inheritance through reminiscence from a previous state of existence. Sundry Christian philosophers of ultra-spiritualist tendencies have described them as innate, planted in the soul at its creation by God. On the other hand, Empiricists and Materialists have endeavoured to explain all our intellectual ideas as refined products of our sensuous faculties. For a fuller account and criticism of the various theories we must refer the reader to any of the Catholic textbooks on psychology. We can give here but the briefest outline of the doctrine usually taught in the Catholic schools of philosophy. Man has a double set of cognitive faculties sensuous and intellectual. All knowledge starts from sensuous experience. There are no innate ideas. External objects stimulate the senses and effect a modification of the sensuous faculties which results in a sensuous percipient act, a sensation or perception by which the mind becomes cognizant of the concrete individual object, e.g., some sensible quality of the thing acting on the sense. But, because sense and intellect are powers of the same soul, the latter is now wakened, as it were, into activity, and lays hold of its own proper object in the sensuous presentation. The object is the essence, or nature of the thing, omitting its individualizing conditions. The act by which the intellect thus apprehends the abstract essence, when viewed as a modification of the intellect, was called by the Schoolmen species intelligibilis; when viewed as the realization or utterance of the thought of the object to itself by the intellect, they termed it the verbum mentale . In this first stage it prescinds alike from universality and individuality. But the intellect does not stop there. It recognizes its object as capable of indefinite multiplication. In other words it generalizes the abstract essence and thereby constitutes it a reflex or formally universal concept, or idea. By comparison, reflection, and generalization, the elaboration of the idea is continued until we attain to the distinct and precise concepts, or ideas, which accurate science demands.


It is important to note that in the Scholastic theory the immediate object of the intellectual act of perception is not the idea or concept. It is the external reality, the nature or essence of the thing apprehended. The idea, when considered as part of the process of direct perception, is itself the subjective act of cognition, not the thing cognized. It is a vital, immanent operation by which the mind is modified and determined directly to know the object perceived. The psychologist may subsequently reflect upon this intellectual idea and make it the subject of his consideration, or the ordinary man may recall it by memory for purposes of comparison, but in the original act of apprehension it is the means by which the mind knows, not the object which it knows — "est id quo res cognoscitur non id quod cognoscitur". This constitutes a fundamental point of difference between the Scholastic doctrine of perception and that held by Locke, Berkeley, Hume, and a very large proportion of modern philosophers. For Locke and Berkeley the object immediately perceived is the idea. The existence of material objects, if we believe in them, can, in their view, only be justified as an inference from effect to cause. Berkeley and idealists generally deny the validity of that inference; and if the theory of immediate perception be altogether abandoned, it seems difficult to warrant the claim of the human mind to a genuine knowledge of external reality. In the Scholastic view, knowledge is essentially of reality, and this reality is not dependent on the (finite) mind which knows it. The knower is something apart from his actualized knowing, and the known object is something apart from its being actually known. The thing must be before it can be known; the act of knowledge does not set up but presupposes the object. It is of the object that we are directly conscious, not of the idea. In popular language we sometimes call the object "an idea", but in such cases it is in a totally different sense, and we recognize the term as signifying a purely mental creation.


There remains the problem of the validity, the objective worth, of our ideas, though this question is already in great part answered by what has gone before. As all cognition is by ideas, taken in their widest signification, it is obvious that the question of the validity of our ideas in this broad sense is that of the truth of our knowledge as a whole. To dispute this is to take up the position of complete scepticism, and this, as has often been pointed out, means intellectual suicide. Any chain of reasoning by which it is attempted to demonstrate the falsity of our ideas has to employ ideas, and, in so far as it demands assent to the conclusion, implies belief in the validity of all the ideas employed in the premises. Again, assent to the fundamental mathematical and logical axioms, including that of the principle of contradiction, implies admission of the truth of the ideas expressed in these principles. With respect to the objective worth of ideas, as involved in perception generally, the question raised is that of the existence of an independent material world comprising other human beings. The idealism of Hume and Mill, if consistently followed out, would lead logically to solipsism, or the denial of any other being save self. Finally, the main foundation of all idealism and scepticism is the assumption, explicit or implicit, that the mind can never know what is outside of itself, that an idea as a cognition can never transcend itself, that we can never reach to and mentally lay hold of or apprehend anything save what is actually a present state of our own consciousness, or a subjective modification of our own mind. Now, first, this is an a priori assumption for which no real proof is or can be given; secondly, it is not only not self-evident, but directly contrary to what our mind affirms to be our direct intellectual experience. What it is possible for a human mind to apprehend cannot be laid down a priori. It must be ascertained by careful observation and, study of the process of cognition. But that the mind cannot apprehend or cognize any reality existing outside of itself is not only not a self-evident proposition, it is directly contrary to what such observation and the testimony of mankind affirm to be our actual intellectual experience. Further, Mill and most extreme idealists have to admit the validity of memory and expectation; but, in every act of memory or expectation which refers to any experience outside the present instant, our cognition is transcending the present modifications of the mind and judging about reality beyond and distinct from the present states of consciousness. Considering the question as specially concerned with universal concepts, only the theory of moderate realism adopted by Aristotle and St. Thomas can claim to guarantee objective value to our ideas. According to the nominalist and conceptualist theories there is no true correlate in rerum naturâ corresponding to the universal term. Were this the case there would be no valid ground for the general statements which constitute science. But mathematics, astronomy, physics, chemistry, and the rest claim that their universal propositions are true and deal with realities. It is involved in the very notion of science that the physical laws formulated by the mind do mirror the working of agents in the external universe. But unless the general terms of these sciences and the ideas which they signify have, corresponding to them, objective correlatives in the common natures and essences of the objects with which these sciences deal, then those general statements are unreal, and each science is nothing more than a consistently arranged system of barren propositions deduced from empty, arbitrary definitions, and postulates, having no more genuine objective value than any other coherently devised scheme of artificial symbols standing for imaginary beings. But the fruitfulness of science and the constant verifications of its predictions are incompatible with such an hypothesis.

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