Knowledge, Theories of

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Theory of knowledge is an area of philosophical speculation concerned with the nature, conditions, and/or first principles of knowledge in general and also, according to some authors, with the truth-value, or reliability, of knowledge in general. The expression "theory of knowledge" is used interchangeably with the term epistemology by some authors, but others, particularly in Europe, mean something else by the latter termusually a critique of modern scientific knowledge (cf. Enciclopedia filosofica 1:1942). Theory of knowledge is among the more controversial areas in philosophy, there being serious disagreement among the different philosophical traditions over the selection and correct formulation of the problems to be considered. Disagreement exists also over the question of whether theory of knowledge should precede and control, or follow and be controlled by, metaphysics and psychology.

Scholastic philosophers generally prefer the second alternative, both because their conception of being and knowing requires that a theory of knowledge rest on something more basic than itself, and also because there is a strong tendency, discoverable in history, for the first alternative to lead to some kind of skepticism.

This article traces the historical development of theories of knowledge from early times to the present, treating successively of the Greek origins of the problem, medieval theories of knowledge, knowledge in modern thought, and contemporary views of knowledge. Systematic analyses, from the viewpoint of scholastic philosophy, of various problems associated with the theory of knowledge are treated elsewhere (see knowledge; sense knowledge; certitude; truth).

Greek Origins of the Problem

Like many problems in philosophy, the problem of knowledge received its earliest formulation and a variety of solutions among the Greeks, the most important of whom include Parmenides, Democritus, Socrates, Plato, and Aristotle.

Parmenides. Questions concerning the nature and conditions of knowledge first assumed importance in the philosophy of parmenides (fl. c. 485 b.c.). Parmenides's predecessors had concerned themselves, from the very beginning of greek philosophy (c. 585 b.c.), with cosmological questions concerning the basic material from which the familiar things of experience are made and concerning the process whereby such things undergo change. That there is a plurality of things undergoing change these thinkers took as an obvious fact. The starting point of Parmenides's thought seems also to have been this plurality, but he found that a more basic affirmation about reality is that it exists. Being is and nonbeing is not. Determined to discover the ultimate implications of this intuition, Parmenides said that being cannot come to be (since being "is" and what "is" cannot come to be) and must therefore be eternal, in the sense of beginningless. Moreover, change must be an appearance only, since change means becoming, and neither being (because it simply "is") nor nonbeing (because it is nothingness) can become. In a conclusion that influenced the subsequent history of Greek thought about being and knowledge, Parmenides declared that being is eternal and changeless.

This intuition controls Parmenides's theory of knowledge. Corresponding to the duality that he introduces between being (or true reality) and appearances, he also introduces a duality between knowledge, the object of which is being and in which alone is to be found truth, and another, corrupted or limited kind of cognition called doxai (Gr. δόξαι), a term often translated, somewhat misleadingly, as "opinions," that rather means one's perceptions of the plural and changing appearances of being. A similar duality is introduced by Parmenides between reasoning, which, because it achieves knowledge about true reality, is the higher way; and sensation, which, tied down to the appearances of plurality and change, leads to perplexity and poor discernment. The ordinary cognition of all men is one in which knowledge and doxai are mixed, and it is only in rare moments of inspiration and illumination that mena few menhave cognitions in which things are seen from the standpoint of the timelessness and changelessness of being.

Atomists and Sophists. The theory of knowledge of the Greek atomistic school (5th century b.c.) was a continuation of Parmenides's doctrine on knowledge, in spite of the fact that, at first glance, a world conceived in terms of a plurality of atoms would seem to have little in common with the Parmenidean world. The best known representative of this school, democritus of Abdera, distinguishing between reality (eternal and changeless atoms moving through the void) and appearances (the changing configurations of atom-groups making up the familiar world), concluded that, since no one perceives the atoms, knowledge of reality is impossible. One has only doxai, which were explained as private sensations resulting from atoms impinging upon the cognitive organs.

While Democritus's theory of knowledge restricted cognition to doxai, his Parmenidean background was strong enough to move him to regard such cognition as superficial and second best; a genuine knowledge of true reality, were it possible, would be better. It was left for the sophists to take the logical step of declaring that, if doxai alone constitute the cognition possible for man, then there is hardly any basis for depreciating and regarding as second best this kind of cognition. Among the Sophists, Protagoras (c. 49 to 420 b.c.) eliminated the Parmenidean notion of truthi.e., stable knowledge of eternal, changeless beingon the ground that it was useless and therefore quite irrelevant in the matter of living one's life wisely and well. Truth, therefore, if it is to be found anywhere, will have to be found in one's everchanging doxai, one's perceptions of the appearances. This doctrine was taken by some to mean that the way in which things happen to appear to an individual is the way they actually are, for him, and that therefore truth is relative to each individual.

Socrates, Plato, and Aristotle. Against this background, socrates, whose chief interest was virtue (identified, for him, with knowing how to live wisely and well), saw that the Sophists' notion of truth made virtue impossible, since ever-changing doxai provided no dependable guides for living well. Accordingly, Socrates sought to discover from the changing, particular appearances of things some permanent and universally valid meanings in terms of which there could be a genuine knowledge of being and hence some basis for making wise decisions.

Socrates's project was carried further by his major disciple plato, who agreed that doxai were not a satisfactory guide for living well. Only genuine knowledge could be a satisfactory guide. Accepting the Parmenidean doctrine that knowledge means knowledge of being and that being is eternal and changeless, Plato concluded that behind the familiar world of changing sensible things appearancesthere is an archetypal world of Forms (Ideas) which, eternal and changeless, provide the stability needed for objects of knowledge. In spite of the everchanging character of the material world about which there can be only doxai, genuine knowledge of permanent and universally valid meaningsand therefore a basis for making wise decisionsis possible, because of the existence of the Forms. These Forms were said by Plato to be known by the soul prior to its imprisonment in the body, and during its earthly life the soul's knowledge is simply recollection.

aristotle, while agreeing with Parmenides and Plato that stability is a necessary requisite for an object of knowledge, rejected Plato's tendency to locate this stability in a separate world of Forms and insisted that, since all knowledge begins with sensible things, there must be something stable in these themselves. This was explained in terms of the Aristotelian hylomorphic doctrine, according to which every sensible substance is a composite of a determinable principle (matter) in virtue of which the substance can change, and a determining principle (form) in virtue of which the substance is what it is (see matter and form). Accordingly, while a sensible substance can change, nevertheless to the extent that it "is" it is stable; hence it is being and is knowableits stability, being, and knowableness resulting from its form. The knower's knowledge was explained by Aristotle in terms of the knower abstracting the form of the known object, so that the knower's knowledge is not something that represents the known object; it actually is the known object (see abstraction).

Later Greeks. Among later Greek philosophers, the Stoics viewed knowledge as consisting in impressions on the soul brought about through sense perceptions of particular things. The Epicureans located truth basically in sensation, which they reduced, after the manner of Democritus, to particles striking the cognitive organs. The Skeptics repudiated all claims to knowledge and truth, but their skepticism was aimed at destroying philosophies and not at paralyzing practical life.

See Also: skepticism; pyrrhonism; cynics; stoicism; epicureanism; neoplatonism.

Bibliography: General introductions. a. h. armstrong, An Introduction to Ancient Philosophy (Westminster, Maryland 1957). j. burnet, Greek Philosophy: Thales to Plato (London 1914; reprint 1932). f. c. copleston, History of Philosophy: v.1: Greece and Rome (Westminster, Maryland 1946; 2d ed. 1950). j. owens, A History of Ancient Western Philosophy (New York 1959). j. i. beare, Greek Theories of Elementary Cognition from Alcmaeon to Aristotle (Oxford 1906). Particular philosophers. g. vlastos, "Parmenides' Theory of Knowledge," American Philological Association, Transactions and Proceedings 77 (1946) 6677. Plato's Theory of Knowledge, tr. f. m. cornford (London 1935). n. gulley, Plato's Theory of Knowledge (New York 1962). w. d. ross, Plato's Theory of Ideas (Oxford 1951); Aristotle (5th ed. London 1953) 136165, 21521. m. m. patrick, The Greek Sceptics (New York 1929). m. de corte, La Doctrine de l'intelligence chez Aristote (Paris 1934).

[f. r. ellis]

Medieval Theories of Knowledge

From such origins in Greek philosophy, theories of knowledge were extensively developed by the early medievals and the schoolmen. Under the sway of Platonism, writers such as St. Augustine, Boethius, St. Anselm, and Abelard set the stage for the later scholastic theories. Aristotle's influence, transmitted by Arab thinkers such as Avicenna and Averroës, added a further dimension to medieval thought. These currents fused in the hochscholastik period, and brought forth various solutions to the problem of knowledge, as proposed principally by St. Bonaventure, St. Thomas Aquinas, John Duns Scotus, and William of Ockham.

Christian Platonists. Like Aristotle, medieval scholars inherited a noetic that derived from Plato and, often enough, succeeded in becoming Aristotelians without altogether ceasing to be Platonists. Knowledge, for instance, was specified by its certitude. The stability of knowledge, they were tempted to think, must be founded on the object known, but the changing singular seemed a poor candidate for the role. Still, permanent structures are discerned in things. To grasp their essences, a Platonic mind has but to concentrate forces that are weakened when dispersed among sensibles. A Platonic being is intelligible of itself and hardly needs modification by the intellect. What is required is rather an ethical purification of the one who knows.

St. Augustine. St. augustine surmounted a crisis of skepticism in his youth with the reflection that to err is to exist and to reject an error is to proclaim truth (Civ. 11.26; Lib. arb. 2.3.7). He urged the mind in search of true knowledge to turn from deceptive externals and to collect her forces, to enter within herself where the mind must recognize that she is under the rule of a higher light (Vera relig. 39.72), that truth is the mind's discovery, not her construction (Doctr. christ. 2.32.50; Lib. arb. 2.12.34; Civ. 11.25), and that this higher light, one for all minds, inferior to nothing, and eternally immutable, is an illumination that flows from and reveals God (Lib. arb. 2.13.3637). Although St. Paul is an indispensable guide into the mysteries of ultimate truth (Conf. 7.21.27), Plato can teach man to turn within and to allow love, the "weight" of the soul (Epist. 157.2.9; Conf. 13.9.10), to bear her above the dissipation of sensual involvement. Sensation is much more a matter of the soul's attention than of corporeal passivity (Musica 6.5.9).

St. Augustine proposes a crucial role for memory. With mind and will, a Trinitarian vestige in man, memory renders the eternal exemplars accessible to the mind (Trin. 12.15:25). Citing Vergil to show that there can be a memory of the present (ibid. 14.11.14), Augustine holds that by memory man retains the past and also sees the present in the light of the exemplars to which the apex of the soul is always present. But he is careful to disclaim a prior existence of the human soul (Civ. 11.23; Retract. 1.4.4).

Boethius. boethius claimed that he had cultivated what Augustine had sown (Trin. ), but to say nothing of his personal genius, he added to his knowledge of Plato a technical training in the philosophy of Aristotle and, despite (or perhaps because of) his knowledge of both, thought they could be reconciled (Lib. de interp. ed. 2.2). Knowledge is proportioned to the capacity of the knower (Consol. 5 prosa 4, 6) and also to the object known (In Porphy. dial. 1; Trin. ), for knowledge is possible only to the extent that knower and known can coincide in their grade of being. The object of man's highest knowledge is the "intellectible," a term coined by Boethius to designate what is one and the same of itself, always consistent in its own divinity, and never grasped by sense, but only by mind and intellect (In Porphy. dial. 1).

The second level of knowledge and thus of being is that of the "intelligibles." Here man proceeds "by thought and understanding concerning the First Intellectible" to grasp the causes of the sublunar sphere and the human soul itself, that soul which, at first an "intellectible," has lost caste by the contamination of body and has become an "intelligible" (ibid. ).

A third level of speculative knowledge discerns the natures and attributes of bodies by prescinding from their matter (ibid. ). God, the object of theology, the supreme degree of speculative knowledge, is approached by man in "intellectual" fashion (intellectualiter ). Intelligible objects, superior to bodies although in contact with them, are approached by way of "discipline" (disciplinaliter ), the rationalizing proper to man; a procedure that results in mathematics. Corporeal natures, "rationally" (rationaliter ) considered, are the objects of physics (Trin. ). Because he has fallen, man cannot know without rationalizing; but because he is a fallen god, an "intellectible," he need not despair of wisdom.

In sensation, the soul is active (Consol. 5.4 prosa 5) and, taken to the letter, Boethius more than once invoked the preexistence of souls to explain knowledge (Consol. 3.11 prosa 12; In Porphy. dial. 1). But these remarks are in the spirit of Platonic myth-making, inspired by man's present state, rather than descriptions of the historic past.

Erigena. One of the most daring applications of dialectic to Christian faith is that of john scotus erigena: since to know God is salvation, ignorance must be the same as damnation (De praed. 17.9). Because nature is "everything that is and everything that is not" (De div. nat. 1), the knowledge of nature is radically theological. "Nature creating, but not created" is God, the divine Ideas are "nature creating and created." The World is "nature, not creating, but created" and the goal of the great return, when God will be all in all, is God qualified as "nature neither creating nor created" (ibid. ). Thus all knowledge, with all being, is treated under the title "On the Division of Nature."

St. Anselm. By his use of the old truth that God is "that than which a greater cannot be conceived" to establish that God necessarily exists, anselm of canterbury posed a sign of contradiction for all succeeding speculation. For those who accept his world, the Proslogion argument is irrefutable, but for those who live in another world, the reasoning remains unconvincing (see ontological argument). This means less than Anselm had a strange theory of truth and knowledge, than that his consciousness of reasoning in the presence of truth and its participations rendered all theories superfluous. Unparticipated truth is being itself, and Anselm's world is no more a world of beings than a world of truths.

Peter Abelard. abelard came to the problem of knowledge impressed by the radical singularity of each individual and by man's inability to recognize in a general concept the individuals of which, nonetheless, it can be predicated. Man knows things, it seemed to him, in three ways: by sensation, when a thing is present; by imagination when, in its totality, a thing is the object of the soul's attention; and by intellection, when the soul attends to some detailed aspect of that thing. Like an artist, the knower holds his material with one hand and shapes it with the other. He works over what is grasped by sense or imagination to discern the forms that exist together, but that can be thought apart. Sensation requires corporeal instruments, but is primarily the work of the soul (Logica 'Ingred.,' Gloss. in Perih. ).

The Arabian Aristotle. With access to Aristotle, there was a shift in the problematic. No longer deplored as an obstacle to knowledge, sensation was accredited as its sole starting point. At once receptive to what sense can deliver and active in dematerializing that content to fit it for intellectual assimilation, intellect implies two powers. As capable of adaption to the forms of other things, intellect is passive. As capable of rendering the potentially intelligible material singular actually intelligible, intellect is "agent." This much is surely in Aristotle, but where did the Stagirite intend to locate these powers?

Arabian commentators took firm positions where the text of Aristotle left some latitude. The intelligence responsible for providing the human knower with suitably immaterial forms is an astronomical deity, styled by Avicenna the "Giver of Forms" or the "Intellect in effect" and by Averroës, the "Agent Intellect." Both hold it is one for all men.

Avicenna. The soul may be, as Avicenna would have it, the very essence of man (De anim. 1.1), but for all Aristotelians, soul is the form of the body, and here Avicenna betrays some uneasiness with Aristotle. For to be a form of matter, Avicenna remarked, defines a function of the soul rather than its nature (ibid. ). As in potency to receive such forms, the intellect of man is "material," by which he meant "passive," not "corporeal." As possessed of forms, but not adverting to them, intellect is "habitual"; as actually knowing that it knows, intellect is "in effect," or, in terms that remind one that intelligible forms come to man from above, it is "borrowed" (accommodatus ) and "acquired" (adeptus ). "The Giver of Forms, totally and always 'in effect,' is to our minds what the sun is to our sight." Not only the source of intelligible forms for human intellects, the Giver of Forms has a second right to his title. This Intelligence provides matter under the sphere of the moon with the constituent forms it is prepared to receive and thus brings new beings into existence.

Averroës. Averroës agreed with Avicenna that there is a "passive" intellect by which a man is capable of receiving intelligible forms from above, but this he described as corporeal imagination, destined to perish at the death of the individual who possesses it. The union of the separate Agent Intellect with the passive one engenders a third intellect that Averroës termed "material," since it too is passive. This third intellect is as little the possession of the individual as light reflected from a body is part of that body. Hence there is no ground in the uncorruptibility of intellect for the immortality of the human individual. All that is individual is corruptible, and all that is incorruptible is both radically separate from matter and one for all men. As the Agent Intellect is one for all men, so too the material intellect is one for the entire race. The highest cognitive faculty that pertains to the individual is the corporeally rooted, and therefore perishable, "passive intellect" or "imagination."

If this is the last word of philosophy, then it is a wisdom incompatible with Christian faith in personal immortality. The "prophetic intellect," source of the illumination that has resulted in that "miraculous" work, the Qurān, reinforces the decision of modern scholarship that the Three Impostors (Moses, Jesus, and Muammad), attributed to Averroés, is a forgery, but it was not enough to defend him from persecution by the Islamic theologians of his own day (In de anima 3.4, 5). see intellect, unity of; double truth, theory of; arabian philosophy.

Aristotle and the Christians. The dangers implicit in the Arabian development of Aristotelian thought were quickly recognized by the schoolmen of the 13th century. Correctives were soon forthcoming, with various repudiations of Aristotle and with a pronounced revival, in some quarters, of the doctrines of St. Augustine.

St. Bonaventure. His intimate knowledge of Aristotle's text notwithstanding, St. bonaventure was little inclined to abandon the pathways of his Christian masters for those of Aristotle. The content of knowledge as garnered through sensation and the intellect can be described in Aristotelian terms, but certitude specifies genuine knowledge and this, as St. Augustine knew and Aristotle did not know, is the fruit of a divine illumination. Neither the created mind that knows, nor the created object known, can be the source of the universality, necessity, and immutability of "certitudinal" knowledge. The divine attributes these terms evoke are the ultimate ground of knowledge (In 2 sent.

Faced with the two-edged risk of ascribing too much to creatures or too much to God, Bonaventure never hesitated to follow the dictates of his piety and to choose the explanation that gives most to God. But not everyone, not even every Franciscan, in the 13th century was content with this solution. Is the divine illumination the same as the general concurrence of God with creatures, or is it a special help? Does divine illumination pertain to the order of nature or to the order of grace? The Franciscans found it increasingly difficult to know with Aristotle and to be certain with Augustine, and the temptation was not always resisted to transfer the problem from philosophy to theology.

St. Thomas Aquinas. The most important of those who declined to accept a piety that exalts the Creator by positing intrinsic deficiencies in creation was St. thomas aquinas. With the balance that is one of his chief glories, however, he admitted that whether, with Plato and Augustine, one says that the intelligibles are participated from God, or, with Aristotle and himself, that what is participated is the very light that renders things intelligible, "does not matter much" (De spir. creat. 10 ad 8). The light of reason implanted in man by God, the natural power of the human mind that Aristotle had called the "agent intellect," is "as it were, a certain similitude of Uncreated Truth" (De ver. 11.1). As Augustine had found much to christen in Plato, Aquinas found the Aristotelian panoply of knowledge within the created structure of man. Intellect, with its passive and active powers, belongs within the human soul. As truly the form of body for Aquinas as for Aristotle, the soul is the single form of man's being and man is profoundly one, for all his wealth of powers. The human soul is by nature incorruptible and a being (hoc aliquid) in its own right, destined to inform a body, but capable of surviving the dissolution of death because by nature incapable of dissolution.

On the other hand, Aquinas had many reservations on how much man can know. His acceptance of the Aristotelian cosmos, for instance, is provisional; although this is a good account of how things seem (apparentia salvarentur ), men may find another that will do as well. For all his "demonstrations," Aristotle was handling as truths what are but hypotheses (In 2 cael. 17). In philosophy, where, in principle, the human intellect "penetrates to the essence" (Summa theologiae 1a2ae, 31.5), the essential principles, substantial formsindeed, the essence of even a flyall remain in fact unknown to man (In 1 anim. 1.15; De spir. creat. 11 ad 3; Symb. 1). The theologian too must resign himself to a modest accomplishment: the most profound moment in his knowledge of God is the realization that men are ignorant of Him (De pot. 7.5 ad 14). Within these limitations, content with knowledge consonant with man's limited being, Aquinas developed both philosophical and theological knowledge.

His successors were less patient. Removing from knowledge whatever fails to meet the highest standard of certitude and working under the shadow of the Parisian condemnations of 1270 and 1277, they relinquished one proposition after another and assigned to belief what they had thought could be known. To preserve what knowledge might be salvaged, they set out on a road that could end only by restricting knowledge to immediate experience.

Scotus and Ockham. John duns scotus accepted Aristotelian abstraction, but his "absolute quiddities," known to be real because they move the intellect, a thing that nonbeing cannot do, are traces, it has been said, of the eternal "reasons" of Augustine (Gilson, 766). To his mastery of Aristotle's theory of knowledge, Scotus added a distinction between man's intuitive knowledge of what exists as such and his abstractive knowledge of common natures that, of themselves, remain indifferent to existence. william of ockham borrowed this terminology, but opposed the doctrine.

For Scotus, to have intuitive knowledge of a nonexistent is a contradiction (Rep. Par., whereas for Ockham, the two knowledges differ intrinsically (seipsis different ) and it is within the absolute power of God to cause intuitive knowledge of what does not exist in man. This is Ockham's way of saying that it is not a contradiction to have intuitive knowledge of a nonexistent (Quodl. 6.6). Abstractive knowledge cannot be concerned with common natures, for the Ockhamist reason that they are gratuitous constructions, unfounded in the real world. Ockham reserves abstractive knowledge for man's grasp of the objects he represents to himself in their absence. Owing to the absolute divine power, even intuitive knowledge is open to the danger of error. If this is so, man's last resource is not even theology. It is faith, as faith was Ockham's final resort. Because faith and knowledge are not identical, their marriage had been possible, but now men began to refuse the name of knowledge to whatever falls short of the absolute certitude possibly only to Absolute Spirit (Pieper, 145). Noetic fatigue could hardly go further.

See Also: dialectics in the middle ages; universals.

Bibliography: É. h. gilson, History of Christian Philosophy in the Middle Ages (New York 1955). j. pieper, Scholasticism, tr. r. and c. winston (New York 1960). r. p. mckeon, ed. Selections from Medieval Philosophers, 2 v. (New York 192930). william of ockham, Philosophical Writings, ed. p. boehner (New York 1957).

[e. a. synan]

Knowledge in Modern Thought

Modern philosophy, commonly regarded as having begun with René Descartes (15961650), developed theories of knowledge having all the characteristics of the new scientific age. Descartes and G. W. Leibniz tried to reduce the complexities of human understanding to basic formulae; British empiricists tried to restrict human knowledge to only what could be measured and empirically observed. Even Immanuel Kant was deeply impressed by the creative scientific hypothesis that apparently brought understanding of reality and explained experience without being based on it; and, in his elaborate theory of knowledge described in The Critique of Pure Reason (1781), Kant presented human knowledge as taking place largely through a priori forms that shape the human understanding and differentiate it from the divine. For G. W. F. Hegel consciousness evolving in rich complexity ultimately constitutes the divine; and this theme was developed in the 19th century by A. Schopenhauer and others.

Descartes and Leibniz. In the philosophy of René descartes, the act of knowledge is central. Seeking a solid basis for his entire system and through a methodic doubt questioning the existence of everything he could not know with certitude, he finally selected his consciousness of thinking as the most self-evident and irrefutable principle possible. "I think, therefore I am." But, if his own existence was implied by his thought, then he must be a substance whose whole nature or essence was thought. "I realized," he writes, "that this substance of myself had no need of a place or any material thing in order to exist. The result was that I,that is, the soul by which I am what I amis entirely distinct from my body." Unsettled too by the obscurity of sensation, Descartes rejected the reports given by the senses about the world. Man must believe the world is real; he possesses no demonstrable certitude of its reality.

Gottfried Wilhelm leibniz continued the same philosophical development. Discoverer of the infinitesimal calculus, Leibniz was attracted by the rigor of the mathematical method of deduction from definitions. Why, he asked, cannot this method be applied to metaphysics as well as to mathematics? Asserting that man's notions of substance, cause, and unity arise out of interior reflection (rather than from sense experience), Leibniz thought that all knowledge of the world could be explained if one conceived of all bodies as being composed of monads, i.e., immaterial, animated units depending directly upon God for existence. The soul and body, although composed of different kinds of monads, work in harmony, forming one being. Man's concept of space arises from the thought of monads coexisting in some relationship; his concept of time arises from the notion of events happening together or successively. Briefly, the world could be best understood, not through sense experience, but through reflection upon the nature of the monad and its infinite possibility of order and perfection under divine direction.

Hobbes and Locke. The British empiricists reacted against such reasoning. To Thomas hobbes, all philosophy begins with sense experience, and philosophy's only function is to explain it. Universal ideas cannot be abstracted from experience; they are simply common names standing for particular things. Colors, sounds, odors, and other qualities are subjective and nonscientific, since they have significance only to an individual, stimulated sensory organ. The "objective" world of bodies in motion is meaningful since they can be measured. Surprisingly, Hobbes speaks of human willing as a form of motion, and describes degrees of human intelligence as traceable to differences in dynamic forces.

Another Englishman, John locke, shared Hobbes's conviction that philosophy should begin with sense experience, but, unlike Hobbes, he did not think that it should end there. Making the nature of human knowledge the central focus of his philosophy (his principal work was Essay concerning Human Understanding ), Locke denied that any ideas are innate. Man's concepts of space and time arise from experience; and although color, sound, and other sensations are not simple reproductions of identical qualities in bodies around him, nevertheless they are objective since they are caused by such bodies. Man can at least infer the existence of bodies from such sensations and call them the unknown substrate of accidents, since he cannot be certain to what extent the substances that cause these sensations possess the qualities corresponding to them. Locke extended this principle to intellection: one knows only his ideas of objects directly, and can merely infer the existence of objects from the fact that he has ideas of them. But how can one be certain that his ideas correspond with their objects? How can he be certain that there are any objects at all? These were the questions posed by his critics.

Berkeley and Hume. Anglican Bishop George berkeley answered such questions for himself and his followers by asserting that there are no objects. All of man's ideas come directly from God. There is no need of a world; for if Locke's substance is unknown and unknowable, it is also unnecessary. Esse est percipi ("to be is to be perceived") was the basic theme of Berkeley's philosophy. Human ideas have their correspondent reality in the divine ideas found in the mind of God. Physical laws of nature do not constitute an adequate explanation of experience; they are generalizations but not explanations of what has happened. The ultimate explanation of nature, or rather of the experience that constitutes nature, is to be found in metaphysics, which teaches that God constantly pours ideas into minds and in this sense continually creates the world.

David hume shared Bishop Berkeley's doubts about the reality of substance. If, as Locke says, the substrate of accidents is unknown, why postulate its existence? The ultimate cause of impressions is beyond explanation. "It will always be impossible to decide with certainty whether they arise immediately from the object or are produced by the creative power of the mind or are derived from the author of our being." He felt that all of human knowledge could be reduced to two kinds of sense impressions: faint, general images, and vivid, individual phantasms. Intellectual or abstract knowledge were excluded. Universal ideas are in reality the general terms with which particular sense images are associated.

Hume rejected any kind of a generalization. Since there is no particular sense quality (i.e., a color or sound) to indicate that a being is an effect, this is unverifiable by the senses and should be rejected. All that can be asserted about causality, he argues, is what is observable, and this is that a certain sequence of events can be seen to take place whereby one event follows another. What follows another in time is called an effect. But there is nothing in any object (such as its contingency) that can prove that it is an effect, i.e., caused by another.

Hume considered his own person to be not a substance but a mass of sensations: "When I enter most intimately into what I call myself I always stumble on some particular impression or other." Hence, he concluded that his self is constituted by "bundles or collections of different perceptions which succeed each other and are in a perpetual flux and movement."

Kant's Criticism. Describing himself as being roused from his dogmatic slumber by the writings of Hume, Immanuel kant wrote his masterpiece, The Critique of Pure Reason, explicitly to save science (especially the laws of mathematics and physics) and morality in general. Admitting that knowledge begins with sense experience, Kant denied that it must end there. In fact (he argues) all of sense experience is conditioned by space and time. Being the condition of sense experience, space and time cannot themselves be explained by such experience. They must be a priori forms of human sensation. For this reason, he continues, metaphysics, or the knowledge of being as it is in itself, should be based not on experience but rather on what conditions experience in human understanding. In The Critique of Pure Reason, Kant attempted to effect what he called a Copernican revolution in philosophy: truth would henceforth be considered as the conformity, not of the intellect with an object, but of the object with the intellect. In other words, philosophy would concern itself not with making certain that its concepts corresponded with an objective reality; its task would be to see that human experience corresponded with the nature of concepts and categories that constitute human understanding.

A later work on morality, The Critique of Practical Reason (1788), insisted that since one cannot discover any objective moral law in nature, he must be guided by an innate concept of holiness, expressed in the maxim "So act that the maxim of your will can always be valid as a principle making universal law."

Hegel and Schopenhauer. Of the great German thinkers following Kant, Georg Wilhelm Friedrich hegel was undoubtedly the greatest If, as Kant averred, consciousness must conclude that thought is purely formal and regulative, should one not go further and maintain that intelligence itself must be considered as absolute? If one cannot know the thing in itself (das Ding an sich ) as Kant was forced to admit, why should he concern himself with it? Cannot consciousness supply its own content, as well as its own form? Thus, the ultimate duality between thought and being, between subject and object can be dissolved. In a transcendent identity of thought and being, of subject and object, would arise spirit (Geist ), which, containing contradictory ideas, transforming them by generating their opposites, would finally synthesize them in a higher form including both. This Hegeltan triad constituting a dynamic dialecticK. marx was later to borrow and exploit in his own system of materialism (see materialism, dialectical and historical).

Arthur schopenhauer continued the tradition of Hegelian idealism. Conceiving philosophy's task as the explanation of conscious experience, he asserted that principles of becoming govern sense experience; principles of being are the basis of mathematical concepts and constructs; and, finally, principles of knowing form the foundation of logical categories. Hence, all of knowledge is determined by innate principles.

Yet Schopenhauer did not entirely lose the Kantian nostalgia for the world as it is in itself. Although it can never be known, Schopenhauer writes, its existence cannot be questioned by man's deepest instinct. The phenomenal world (the world as it appears) is a world of conflict; the noumenal world (the world as it is in itself) is a world of peace, and the soul yearns for it. Suffering and sympathy lead man away from the phenomenal world to the noumenal world, and the human spirit finds its highest act in heroic self-renunciation.

Twentieth-Century Views

In the 20th century, disillusionment with Hegelian philosophies of the Absolute Spirit gave rise to the study of knowledge as a purely human act. Neo-Kantians abandoned the quest for "the thing-in-itself" (which Kant had said could be known only by a divine mind) and, while insisting that the only meaningful knowledge would be that of an object as it appears, asserted that the highest type of knowledge is the provisional explanation scientists give of reality. Under the leadership of Edmund Husserl, phenomenologists held that judgment about the actual existence of reality can be suspended; it is sufficient to describe simply what appears to consciousness. Existentialists, such as J. P. Sartre (19051980), flatly denied that one can know what a thing is in itself; in itself a thing has no essence. It is sufficient to know that a thing is, for its entire meaning is imposed upon it by man. Logical analysts insisted that no statement is meaningful if its elements cannot be verified by the senses; and since man is only too prone to confuse what he experiences with irrelevant materials drawn from grammar and religious beliefs, all his statements should be purified by being subjected to rigorous logical analysis.

Bergson and Dilthey. Impatient with the self-destructive tendencies in German idealism, philosophers of France were moved to construct systems emphasizing the dynamic aspects of consciousness. To Henri bergson, the essence of reality is not being but becoming. Against Kant, he denied that time is an a priori condition of experience; it is, instead, the very essence of experience. Time should not be excluded from metaphysics; metaphysicians should try to understand the primary role that time plays in human experience and in the universe itself. All matter is in motion; consciousness itself may be said to be constituted by time and motion since it is continually evolving. In fact, all things can be said to be part of the élan vital, the surging flow of life toward higher forms of freedom and consciousness. The ordinary function of intelligence consists in devising means whereby human life can progress; its highest act, however, is the intuition whereby the intelligence understands its relationship with God and its destiny of transcending present limitations.

Eminent German philosophers, such as Wilhelm dilthey, had similar philosophical ideas. Dilthey reacted against Kantian and Hegelian philosophy as being systems of static concepts, and characteristically suggested that philosophy should be viewed as a form of history, since history alone can express experience as a living whole.

Neo-Kantians. A group of German philosophers at Marburg, calling themselves Neo-Kantians, thought that ordinary knowledge is nonscientific and nonphilosophical. Epistemology should restrict itself to examination and evaluation of the philosophical implications of the methods and statements of positive sciences. Herman Cohen (18421918), the most distinguished proponent of the Marburg school, felt that since scientific explanations of phenomena are based upon intellectual constructs, all reality (i.e., all phenomena) might be reduced to laws of reason. Hence, the ultimate explanation of reality would be found in logic, rather than in metaphysics. Wilhelm Windelband (18481915), founder of the Baden school, developed a similar theory of epistemology, stressing the importance of subjective cultural values in human experience. He maintained that the truth value of judgments is determined by their correspondence with indemonstrable values that the mind has by its nature and expresses in its logic, ethics, art, and religion.

Phenomenologists. Edmund husserl is frequently called the founder of the phenomenological method, which stresses the importance of returning to things themselves. In perception, a thing is present to consciousness. It is a phenomenon; it appears to man. To avoid the conflict between the realists and the idealists, Husserl asserted that it is sufficient to say simply that the thing appears; one can suspend judgment as regards its existence. And since essence determines the meaning of an object, its actual existence is relatively unimportant to consciousness. By describing what appears, the phenomenologist gets at the essence, and this is what matters. (see phenomenology.)

Neorealists and Naturalists. Neorealism did not accept this view, however. Its adherents asserted that it is possible and highly important to affirm trans-subjective reality. In England, Bertrand russell is frequently considered to be a neorealistalthough his views changed considerably over the years. He insisted that philosophy should ask hard, matter-of-fact questions about the data that empirical science provides about existing objects. Philosophy would not be a science at all were it not for the fact that, while clarifying the concepts of empirical science, it constructs a purely formal logic; yet its laws, like those of science, have no more than high probability. In the United States, George santayana was frequently numbered among the neorealists, although his views were quite opposed to those of Lord Russell. Santayana maintained that substance is external to consciousness, and, although constant assumes various shapes that consciousness understands through modifications in space and time. What lies behind space, time, and substance is unknowable.

Alfred North whitehead is sometimes called a realist, for his interest in empirical science and its methods made him sympathetic to Russell's views. However, Whitehead held that philosophy can and should go beyond science. It should embrace all of human experience, including art and religion. Although philosophy must begin with experience, it is not constrained within its limits; actually, even so-called material bodies are but convenient concepts enabling one to explain experience. The world is made up, not of things, but of eventsthe central event being the act of consciousness, which contains the past and anticipates the future.

In the United States, the term naturalist was applied to the school initiated by John Dewey. Defining thought as the reaction of the intelligence to the doubtful as such, Dewey emphasized the nature of thought as inquiry, rather than understanding or contemplation. Thought and learning must be active, rather than passive. To him the methods of empirical science, of affirmation and varification through experiment, were the only valid sources of certainty. Although he had been educated in a strongly Hegelian environment and had been a Hegelian himself for a brief period, he developed a contempt for what he called "idealistic speculation." He considered metaphysics a harmful pastime, and accepted ethical ideals as real only if they succeeded in moving men to constructive action in society. The highest function of human intelligence takes place when, understanding the present, the mind employs itself in the service of society to realize its ideals in the future. No value is absolute or terminal. New and higher values will always appear. Hence, all knowledge must be provisional. Absolute certitude is not only impossible; it is illusory even to desire it.

Neopositivists and Analysts. Neopositivism is a name frequently applied to the philosophy of the Vienna Circle, represented in the United States by Rudolph Carnap (18911970) and Hans Reichenbach (18911953), two German émigrés. Reverencing logic as the philosophical science par excellence, neopositivists restricted the role of philosophy to analysis of scientific methods and procedures. Ludwig wittgenstein, in his Tractatus Logico-Philosophicus (1921), often called the "neopositivist Bible," asserted that universal statements cannot possibly be based upon the intellectual knowledge of natures or intellectual abstraction. They are simply shorthand symbols of many individual facts or events. Consequently, it is meaningless to speak of a metaphysics or ethics arising from a consideration of the nature of being or the nature of man and society. (see logical positivism.)

Closely allied with neopositivism and sometimes identified with it is the analytical school represented by A. J. Ayer (19101989) and Gilbert Ryle (19001976). In Language, Truth and Logic (1936), Ayer proclaims the principle of verification whereby nothing can be considered as true if it cannot be immediately verified by sense experience. The concept of substance is regarded as having arisen out of grammar, i.e., the need to have a subject for predicates; and metaphysics in general is presented as a word game inferior to poetry, since the poet realizes that he is working in the realm of imagination whereas the philosopher does not. Ryle's work, The Concept of Mind (1949), presents philosophy as a system of linguistic analysis whose burden is to clarify general statements. "Philosophy is the replacement of category-habits by category disciplines"; that is, philosophy should concern itself, not with trying to divide being into categories (as Aristotle and Kant attempted to do) but rather with establishing a systematic explanation of the categories themselves.

Existentialists. Another school of thought, existentialism, derives its name from the fact that its proponents stressed the importance of existence over essence. Plato, they alleged, made ideas or essences the supreme concern of philosophers. Almost all philosophers who have followed him have been "essentialists," constructing their systems out of ideas, essences, and definitions. But an essence as such has no reality. The Universal man does not exist. It is men who existexisting individuals, not general essences.

Kierkegaard. Søren kierkegaard is generally acknowledged to be the father of existentialism. Danishborn and inheriting beliefs that man is essentially evil, with an intellect so darkened that he can know nothing of God and His laws except what he learns through blind faith, Kierkegaard attacked Hegelianism and German idealism as empty rationalisms without any relevance to life. Human lives are ruled, not by logic, but by God, whose providence is incomprehensible and whose acts (such as His command to Abraham to kill Isaac) are seemingly absurd. Philosophy should not discourse about natures and essences; this is metaphysical make-believe. The world is constituted of individual existing beings without any intelligible interrelationship.

Jaspers. Karl Jaspers (18831969), a Protestant, can also be classified as an existentialist, although, unlike Kierkegaard, he had no aversion to philosophy as a system, as his three-volume Philosophie (1932) indicates. In the spirit of Kierkegaard, he described the self as being basically consciousness of the self as sin. But, consciousness is existence, and existence is consciousness, since meaningful relationships can exist only through the act of knowledge. Moreover, one cannot talk about subject and object in the act of knowledge as though they had no relationship. What is called object is already assimilated to the subject and has no meaning without the subject. For Jaspers, the term existence includes both subject and object, even though he seems to accentuate the role of subject by describing the manner in which existence exercises liberty through the creation of values.

Marcel. Gabriel Marcel (18891973), regarded frequently as a Catholic existentialist, conceived of human life as a continuing encounter with God, a dialogue beginning in time and enduring in eternity. Without denying that the intellect can know the nature of man, Marcel (like Martin Buber, the Jewish existentialist) insisted that each man is unique and his ultimate meaning to be found in his incommunicable personality, rather than in the nature he shares with others.

Heidegger. Martin Heidegger (18891976) and Jean Paul Sartre were the two most influential 20th-century existentialists. Heidegger, once a Catholic, later an atheist, and still later a theist, outlined his principal ideas in Sein und Zeit (1927). Rejecting the classical inquiry into being as such, Heidegger begins with a study of what he describes simply as Dasein. Although Heidegger himself believed that Dasein defies translation, some felt that its general notion can be conveyed through the phrase "individual consciousness." It is this kind of being that has primacy; all other beings exist for consciousness. It is consciousness that gives meaning to all objects; it makes all serve its purposes and its projects. Moreover, since human consciousness is open to knowledge of all things, its potential enrichment is limited only by death; in fact, consciousness can be described as "freedom until death," i.e., consciousness can become all things until death cuts it short.

Sartre. Overtly atheistic, Sartre similarly centered his system in human consciousness, which he described as "nothingness" in L'Etre et le néant (1943), since consciousness finds its entire meaning in what it knows, and paradoxically is meaningless unless it is consciousness of something other than consciousness. However, the intellect does not come upon a world constituted of fixed essences, since there are no essences but only existents. Every object stands alone. Each object is unique, non-related to other objects, hence absurd. An object can be defined only as the totality of its possible or actual phenomenological aspects; but the phenomenological aspects of any object are determined by the needs, desires, and values of each consciousness.

According to Sartre, the essence of consciousness is becoming. Were it to cease becoming, it would cease to be; and, although this potentiality immanent within consciousness gives it life and movement, it condemns it to the restless existence of being forever a projection into a nonexistent future (for the future as such never is).

Concluding Evaluation. Most historians of philosophy agree that the foregoing disputes over the nature of knowledge have arisen from an over-simplification of human understanding. Constituted as it is of both abstract and concrete elements, man's knowledge is subject to two kinds of analysis and development. The idealists seem to have concentrated on the abstract elements in human knowledge; for them knowledge soon becomes a matter of concepts and definitions. Empiricists, on the other hand, have concentrated on the concrete elements of human knowledge in a laudable effort to "stay in the real world"; but, ironically, their world of reality seems to be without meaning, as the existentialists have been quick to point out.

Any adequate theory of knowledge must, it seems, consider human knowledge as the complex operation of an intellectual substance that is the form of a human body. Unless the human soul had sense organs and formed perceptions from which to abstract its concepts, it could not know any existing thing. Thus, abstract knowledge cannot be viewed entirely apart from its empirical origins; otherwise it will lose its existential significance. On the other hand, empirical knowledge cannot be viewed entirely apart from the common elements that are discoverable within it; otherwise it will lose its full intelligibility.

Scholastic and Greek theories of knowledge do, in fact, have many concerns found also in modern theories. For example, scholastic philosophers are usually careful to point out that the intellect rarely knows the essences of material substances completely. Hence, man's knowledge of such substances is normally provisional, and subject to revision. In his commentaries on the logical writings of Aristotle, St. Thomas Aquinas notes how frequently one must rely upon nominal or provisional definitions, and he expresses as well the continuing need of logical analysis of terms in judgments.

In any event, there are as many theories of knowledge as there are theories of man, for epistemology itself is always shaped by basic views regarding the nature of the human soul and of the reality that surrounds man of which he is a part. (see man; soul, human.)

See Also: agnosticism; conceptualism; criticism; dogmatism; empiricism; exemplarism; fideism; idealism; materialism; nominalism; phenomenalism; positivism; pragmatism; rationalism; realism; relativism; sensism; solipsism; traditionalism.

Bibliography: i. m. bocheŃski, Contemporary European Philosophy, tr. d. nicholl and k. aschenbrenner (Berkeley 1956). j. d. collins, A History of Modern European Philosophy (Milwaukee 1954). History of Philosophy ; v. 4: Descartes to Leibniz (Westminster, Maryland 1958). l. m. rÉgis, Epistemology, tr. i. c. byrne (New York 1959). m. j. adler, ed. The Great Ideas: A Syntopicon of Great Books of the Western World, 2 v. (Chicago 1952); v.2, 3 of Great Books of the Western World 1:880920. p. prini, Enciclopedia filosofica, 4 v. (Venice-Rome 1957) 2:81340. r. eisler, Wörterbuch der philosophischen Begriffe, 3 v. (4th ed. Berlin 192730) 1:38995.

[r. w. mulligan]