EPISTEMOLOGY

the study of the origin, nature, and limits of human knowledge. The name is derived from the Greek words episteme (knowledge) and logos (reason). Epistemology has had a long history spanning the time of the pre-Socratic Greeks up to the present. Along with metaphysics, logic, and ethics, it is one of the four main fields of philosophy, and nearly every great philosopher has contributed to the literature on this topic. The major issue with respect to the origins of knowledge is whether all knowledge is derived from experience. There are two sharply opposed traditions: empiricism, which affirms this view, and rationalism, which rejects it. Rationalists believe there are innate ideas (i.e., concepts man has independent of experience), such as the notion of equality, which are not found in experience. Some rationalists contend that these notions derive from the structure of the human mind, others that they exist independently of the mind and are apprehended by the mind when it reaches a certain degree of sophistication. Empiricists, by contrast, deny that there are any concepts that exist prior to experience, and accordingly they assert that all knowledge is a product of human learning in which perception plays the main role. Perception itself is problematic, however, since visual illusions and hallucinations show that perception cannot always depict the world as it actually is. Another problem for empiricists is the status of mathematical theorems whose truth conditions do not depend on experience and seem to be known a priori ( i.e., prior to experience). The empiricist response to this claim is that mathematical theoerems are empty of cognitive content and merely express the relationship of certain concepts to one another. The great achievement of the 18th-century German philosopher Immanuel Kant was to have worked out a compromise between these competing views. He argued that human beings do have knowledge that is prior to experience and yet is not devoid of cognitive significance, the principle of causality being one such example. Kant's view can be summarized in the maxim that there are a priori synthetic concepts. The issues about the origins of knowledge are connected with questions about its limits. Many empiricists, such as David Hume, and nonempiricists, such as Kant, agree that the human mind has the capacity to generate questions that no possible appeal to experience could answer, such as whether there is a God, whether the world has a first cause or is uncaused, and whether there is a reality behind that apprehended by the senses. Kant labeled such questions transcendental (i.e., going beyond the limits of rational inquiry), and in the 20th century, so-called logical positivists, such as Moritz Schlick, Rudolf Carnap, and A.J. Ayer, have declared such questions to be metaphysical and devoid of cognitive significance. Questions about the nature of knowing span a wide range, including inquiries as to whether knowledge is a type of belief or is different from belief, and whether knowledge is a special faculty in the mind or is a disposition to act in certain ways. There is some measure of agreement in dealing with such questions. Thus it is generally accepted that any analysis of knowledge must satisfy the conditions that if a person A can truly be said to know that p, where p is a proposition, p must be true, and A cannot be mistaken. This characterization connects knowledge with certitude and thus with issues raised by ancient and modern Skeptics, such as whether a person can achieve certitude about the world, about the past, about the mind of another person, or about oneself. the study of the nature, origin, and limits of human knowledge. The name is derived from the Greek episteme (knowledge) and logos (reason), and accordingly the field is sometimes referred to as the theory of knowledge. Epistemology has had a long history, spanning the time from the pre-Socratic Greeks to the present. Along with metaphysics, logic, and ethics, it is one of the four main fields of philosophy, and nearly every great philosopher has contributed to the literature on the topic. Additional reading General works The texts of the classics mentioned below for which specific editions have not been noted are available in many English-language translations; two notable collections are The Loeb Classical Library and Oxford Classical Text series. The history of epistemology Ancient An excellent collection on skepticism is Miles Burnyeat (ed.), The Skeptical Tradition (1983). For Greek Skepticism in particular, see Charlotte L. Stough, Greek Skepticism: A Study in Epistemology (1969). The chief epistemological works of Plato are his Meno, Theaetetus, and Republic, especially Books VVII. The views of Aristotle can be found in On the Soul, Metaphysics, Book IV, ch. 5 and 6, and Posterior Analytics, Book I, ch. 3. The locus classicus for ancient skepticism is R.G. Bury (trans.), Sextus Empiricus, 4 vol. (193349), in The Loeb Classical Library series. From among the voluminous writings of Augustine, see Against the Academicians, trans. by Mary Patricia Garvey (1942, reissued 1978). Medieval For the period as a whole, see appropriate articles in The Cambridge History of Later Greek and Early Medieval Philosophy, ed. by A.H. Armstrong (1967); and The Cambridge History of Later Medieval Philosophy: From the Rediscovery of Aristotle to the Disintegration of Scholasticism, 11001600, ed. by Norman Kretzmann, Anthony Kenny, and Jan Pinborg (1982). For the thoughts of Anselm Of Canterbury, see his Proslogium, ch. 1, and On Truth. Thomas Aquinas, Summa Theologiae, discusses the soul in general in part I, question 77, and the intellectual powers of the soul in part I, question 79. The views of John Duns Scotus can be found in the relevant sections of his Philosophical Writings, trans. by Allan Wolter (1962, reprinted 1987); and in A.P. Martinich, Duns Scotus on the Possibility of an Infinite Being, Philosophical Topics, supplementary vol. 80, pp. 2329 (1982). The ideas of William Of Ockham can be found in the relevant sections of his Philosophical Writings, trans. by Philotheus Boehner (1957, reissued 1967). Modern Two excellent and now classic histories of early modern philosophy from different perspectives are Edwin Arthur Burtt, The Metaphysical Foundations of Modern Physical Science, rev. ed. (1972), which emphasizes the effect of modern science on philosophy; and Richard H. Popkin, The History of Scepticism from Erasmus to Spinoza, rev. and expanded ed. (1979), which emphasizes the rediscovery of skepticism in the 16th century. Ren Descartes's greatest work is Meditations on First Philosophy, trans. by John Cottingham (1986; originally published in Latin, 1641). John Locke attacks the doctrine of innate ideas in Book I of his An Essay Concerning Human Understanding, ed. by Peter H. Nidditch (1975), while his position on knowledge is developed in Books II and IV. A good introduction to Locke's thought is John W. Yolton, Locke: An Introduction (1985). The best work of George Berkeley is A Treatise Concerning the Principles of Human Knowledge, ed. by Kenneth Winkler (1982); a more popular presentation of his views is Three Dialogues Between Hylas and Philonous, ed. by Robert Merrihew Adams (1979). Daniel E. Flage, Berkeley's Doctrine of Notions: A Reconstruction Based on His Theory of Meaning (1987), discusses a central but neglected aspect of Berkeley's epistemology. David Hume's most expansive discussion of knowledge is in Book I of A Treatise of Human Nature, 2nd ed., edited by L.A. Selby-Bigge and rev. by P.H. Nidditch (1978). A later and more accessible statement of Hume's view is presented in An Enquiry Concerning Human Understanding, ed. by Eric Steinberg (1977). Immanuel Kant, Critique of Pure Reason, trans. by Norman Kemp Smith (1929, reissued 1978; originally published in German, 1781), is Kant's greatest work. The best clear, brief, and accurate explanation of Kant's epistemology is A.C. Ewing, A Short Commentary on Kant's Critique of Pure Reason (1938, reprinted 1987). An important book that rejects the view of Kant as a phenomenalist or subjective idealist is Henry E. Allison, Kant's Transcendental Idealism: An Interpretation and Defense (1983). For G.F.W. Hegel's criticisms of Kant, see his Lectures on the History of Philosophy, trans. from German by Elizabeth S. Haldane and Frances H. Simson (1896, reprinted 1974), part III, section iii, B. A major study on the relationship between Kant and Hegel is Robert B. Pippin, Hegel's Idealism: The Satisfactions of Self-Consciousness (1989). Contemporary A short and readable history of Continental philosophy is Robert C. Solomon, Continental Philosophy Since 1750: The Rise and Fall of the Self (1988). Martin Heidegger, Being and Time, trans. by John Macquarrie and Edward Robinson (1962, reissued 1978; originally published in German, 1927), presents an alternative epistemological scheme. John Dewey, The Quest for Certainty: A Study of the Relation of Knowledge and Action (1929, reissued 1979), is an attack on modern epistemology by an American pragmatist. Richard Rorty, Philosophy and the Mirror of Nature (1979), a history of modern and contemporary philosophy, has attracted great attention as an attack on classical epistemology from an analytically trained philosopher. Paul Feyerabend, Against Method, 2nd ed. (1988), advocates what he describes as an anarchistic theory of knowledge. Karl R. Popper, Objective Knowledge: An Evolutionary Approach, rev. ed. (1979), argues against a tradition that goes back at least to Aristotle and rejects the subjective interpretation of knowledge, according to which knowledge is located in individual people.The 20th-century literature on perception and knowledge is vast. A good general collection is Robert J. Swartz (ed.), Perceiving, Sensing, and Knowing (1965, reissued 1976), which includes two important attacks upon sense-data theoryW.H.F. Barnes, The Myth of Sense-Data, and G.A. Paul, Is There a Problem About Sense-Data?and a strong defense of the sense-datum view by C.D. Broad, The Theory of Sensa. The most important pre-World War II books on perception and knowledge are Bertrand Russell, The Problems of Philosophy (1911, reissued 1988), and Our Knowledge of the External World (1926, reissued 1972); G.E. Moore, Philosophical Studies (1922, reissued 1970), especially the important articles defending sense-data theory, Some Judgments of Perception and The Status of Sense-Data; H.H. Price, Perception (1932, reprinted 1981), which invokes the notion of a sense-datum in defense of the causal theory of perception; and Alfred J. Ayer, The Foundations of Empirical Knowledge (1940, reissued 1971), which merges sense-data theory with the principles of logical positivism. Notable works since World War II include Gilbert Ryle, The Concept of Mind (1949, reprinted 1984), which defends a sophisticated form of epistemological behaviourism; Roderick M. Chisholm, Perceiving: A Philosophical Study (1957); and J.L. Austin, Sense and Sensibilia (1962), which contains a withering assault on the sense-data theory from the standpoint of ordinary-language philosophy. Surfaces, perception, and knowledge are discussed in Thompson Clarke, Seeing Surfaces and Physical Objects, in Max Black (ed.), Philosophy in America (1965); James J. Gibson, The Ecological Approach to Visual Perception (1979, reissued 1986); and Avrum Stroll, Surfaces (1988). The theory of representative realism is given a sophisticated defense in Frank Jackson, Perception (1977); and S. Ullman, Against Direct Perception, Behavioral & Brain Sciences, 3(3):373415 (September 1980), attacks direct realism, especially J.J. Gibson's version of that theory, from a standpoint of modern cognitive science. A comprehensive survey of the literature from about 1980 to 1984 on direct realism and representative realism is to be found in Edmond Wright, Recent Work in Perception, American Philosophical Quarterly, 21:1730 (January 1984).Knowledge and the commonsense view of the world are discussed by G.E. Moore, A Defence of Common Sense, in his Philosophical Papers (1959); Ludwig Wittgenstein, On Certainty, trans. from German (1969); Norman Malcolm, Thought and Knowledge (1977); and John R. Searle, Intentionality: An Essay in the Philosophy of Mind (1983). An excellent simple survey of the impact of computer studies, work in artificial intelligence, neuroscience, and neurobiology on our knowledge of other minds is found in Paul M. Churchland, Matter and Consciousness: A Contemporary Introduction, rev. ed. (1988).Two excellent anthologies are Harold Morick (ed.), Challenges to Empiricism (1972, reprinted 1980); and Paul K. Moser and Arnold Vander Nat (eds.), Human Knowledge: Classical and Contemporary Approaches (1987). Edmund L. Gettier, Is Justified True Belief Knowledge? in Analysis, 23:12123 (June 1963), is considered by many to be a decisive refutation of the justified, true-belief analysis of knowledge. Noam Chomsky, Language and the Problems of Knowledge (1988), discusses innateness, language, and psychology. Roderick M. Chisholm, Theory of Knowledge, 3rd ed. (1989); and Robert Audi, Belief, Justification, and Knowledge (1988), are two good introductions to standard epistemological problems. A.P. Martinich Avrum Stroll The history of epistemology Contemporary philosophy Contemporary philosophy begins in the late 19th and early 20th century. Much of what sets contemporary philosophy off from modern philosophy is its explicit criticism of the modern tradition and sometimes its apparent indifference to it. There are two basic strains of contemporary philosophy: Continental philosophy, which designates the philosophical style of western European philosophers, and Anglo-American, or analytic, philosophy, which includes the work of many European philosophers who immigrated to Britain, the United States, and Australia shortly before World War II. Continental philosophy In epistemology, Continental philosophers during the first quarter of the 20th century were preoccupied with the problem of overcoming the apparent gap between the knower and the known. If a human being has access only to his own ideas of the world and not the world itself, how can there be knowledge at all? The German philosopher Edmund Husserl (18591938) thought that the standard epistemological theories had become intrusive because philosophers were attending to repairing or complicating them rather than focusing on the phenomena of knowledge as humans experience them. To emphasize this reorientation of thinking, he adopted the slogan, To the things themselves. Philosophers needed to recover the sense of what is given in experience itself, and this could only be accomplished through a careful description of phenomena. Thus, Husserl called his philosophy phenomenology, which was to begin as a purely descriptive science and only later to ascend to a theoretical, or transcendental, science. Husserl thought that the philosophies of Descartes and Kant presupposed a gap between the aspiring knower and what is known and that the experience of the external world was thus dubious and had to be proven. These presuppositions violated Husserl's belief that philosophy, as the most fundamental science, should be free of presuppositions. Thus, he held that it is illegitimate to assume there to be any problem of knowledge or of the external world prior to an investigation of the matter without any presuppositions. Husserl's device to cut through the Gordian knot of such assumptions was to introduce an epoche. In other words, he would bracket or refuse to consider traditional philosophical problems until after the phenomenological description had been completed. The epoche was just one of a series of so-called transcendental reductions that Husserl proposed in order to ensure that he was not presupposing anything. One of these reductions supposedly gave one access to the transcendental ego, or pure consciousness. Although one might expect phenomenology then to describe the experience or contents of this ego, Husserl instead aimed at eidetic reduction, that is, the discovery of the essences of various sorts of ideas, such as redness, surface, or relation. All of these moves were part of Husserl's desire to discover the one, perfect methodology for philosophy in order to ensure absolute certainty. Because Husserl's transcendental ego seems very much like the Cartesian mind that thinks of a world but does not have either direct access to or certainty of it, Husserl tried in Cartesianische Meditationen (1931; Cartesian Meditations) to overcome the apparent gap, the very thing he had set out either to destroy or bypass. Because the transcendental ego seems to be the only genuinely existent consciousness, Husserl also tried to overcome the problem of solipsism. Many of Husserl's followers, including his most famous student, Martin Heidegger (18891976), recognized that something had gone radically wrong with the original direction of phenomenology. According to Heidegger's diagnosis, the root of the problem was Husserl's assumption that there is an Archimedean point for human knowledge, to use Husserl's own phrase; but, there is no ego detached from the world and filled with ideas or representations, according to Heidegger. In Being and Time (1927) Heidegger returned to the original formulation of the phenomenological project as a return to the things themselves. Thus, all the transcendental reductions are abandoned. What he claimed to discover is that human beings are inherently world-bound. The world does not need to be derived; it is presupposed by human experience. In their prereflective experience, humans inhabit a sociocultural environment, in which the primordial kind of cognition is practical and communal, not theoretical or individual (egoistic). Human beings interact with the things of their everyday world (Lebenswelt) as a workman interacts with his tools; they hardly ever approach the world as a philosopher or scientist would. The theoretical knowledge of a philosopher is a derivative and specialized form of cognition, and the major mistake of epistemology from Descartes to Kant to Husserl was to take philosophical knowledge as the paradigm for all knowledge. Heidegger's insistence that a human being is something that inhabits a world notwithstanding, he marked out human reality as ontologically special. He called this reality Dasein, the being, apart from all others, which is present to the world. Thus, like the transcendental ego, a cognitive being takes pride of place in Heidegger's philosophy. In France the principal phenomenological proponent of the mid-century was Maurice Merleau-Ponty (190861). But he rejected Husserl's bracketing of the world, that is, his mistake in not recognizing that human experience of the world is primary, a view capsulized in Merleau-Ponty's phrase the primacy of perception. He furthermore held that dualistic analyses of knowledge, such as the Cartesian mindbody dualism, are inadequate. In fact, no conceptualization of the world can be complete in his view. Because human cognitive experience requires a body and the body a position in space, human experience is necessarily perspectival and thus incomplete. Although humans experience material beings as multidimensional objects, part of the object always exceeds the cognitive grasp of the person just because of his limited perspective. In Phenomenology of Perception (1945), Merleau-Ponty develops these ideas (along with a detailed attack on the sense-datum theory, discussed below). The epistemological views of Jean-Paul Sartre (190580) share some features with Merleau-Ponty's. Both reject Husserl's transcendental reductions, and both think of human reality as being-in-the-world. But Sartre's views have Cartesian elements that were anathema to Merleau-Ponty. Sartre distinguished between two basic kinds of being. Being-in-itself (en soi) is the inert and determinate world of nonhuman existence. Over and against it is being-for-itself (pour soi), which is the pure consciousness that defines human reality. Later Continental philosophers attacked the entire philosophical tradition from Descartes to the 20th century for its explicit or implicit dualisms. Being/nonbeing, mind/body, knower/known, ego/world, being-in-itself/being-for-itself are all variations on a way of philosophizing that the philosophers of the last third of the 20th century have tried to undermine. The structuralist Michel Foucault (192684) wrote extensive historical studies, most notably The Archaeology of Knowledge (1969), in order to demonstrate that all concepts are historically conditioned and that many of the most important ones serve the political function of controlling people rather than any purely cognitive purpose. Jacques Derrida has claimed that all dualisms are value-laden but indefensible. His technique of deconstruction attempts to show that every philosophical dichotomy is incoherent, because whatever can be said about one term of the dichotomy can also be said of the other. Dissatisfaction with the Cartesian philosophical tradition can also be found in the United States. The American pragmatist John Dewey (18591952) directly challenged the idea that knowledge is primarily theoretical; experience, he argued, consists of an interaction between a living being and his environment. Knowledge is not a fixed staring at something but a process of acting and being acted upon. Richard Rorty has done much to reconcile Continental and Anglo-American philosophy. He has argued that Dewey, Heidegger, and Ludwig Wittgenstein are the three greatest philosophers of the 20th century, specifically because of their attacks on the epistemological tradition of modern philosophy. The history of epistemology Ancient philosophy Pre-Socratics The central focus of ancient Greek philosophy was its attempt to solve the problem of motion. Many pre-Socratic philosophers thought that no logically coherent account of motion and change could be given. This problem was a concern of metaphysics, not epistemology, however, and in the present context it suffices merely to allude to the arguments of Parmenides and Zeno of Elea against the possibility that anything moves or changes. The consequence of this position for epistemology was that all major Greek philosophers held that knowledge must not itself change or be changeable in any respect. This requirement motivated Parmenides, for example, to hold that thinking is identical with being (what exists or is unchanging) and that it is impossible to think of nonbeing or becoming (what changes) in any way. Plato Plato (c. 427347 BC) accepted the Parmenidean constraint on any theory of knowledge that both knowledge and its objects must be unchanging. One consequence of this, as Plato pointed out in Theaetetus, is that knowledge cannot have physical reality as its object. In particular, since sensation and perception have various kinds of motions as their objects, knowledge cannot be the same as sensation or perception. The negative thesis of Plato's epistemology consists, then, in the denial that sense experience can be a source of knowledge on the ground that the objects apprehended through the senses are subject to change. To the extent that humans have knowledge, they attain it by transcending the information provided by the senses in order to discover unchanging objects. But this can be done only by the exercise of reason, and in particular by the application of the dialectical method of inquiry inherited from Socrates. The Platonic theory of knowledge is thus divided into two parts: a quest first to discover whether there are any unchanging objects and to identify and describe them and second to illustrate how they could be known by the use of reason, that is, via the dialectical method. Plato used various literary devices for illustrating his theory; the most famous of these is the allegory of the cave in Book VII of The Republic. The allegory depicts ordinary people as living locked in a cave, which represents the world of sense-experience; in the cave people see only unreal objects, shadows, or images. But through a painful process, which involves the rejection and overcoming of the familiar sensible world, they begin an ascent out of the cave into reality; this process is the analogue of the application of the dialectical method, which allows one to apprehend unchanging objects and thus acquire knowledge. In the allegory, this upward process, which not everyone is competent to engage in, culminates in the direct vision of the sun, which represents the source of knowledge. In searching for unchanging objects, Plato begins his quest by pointing out that every faculty in the human mind apprehends a set of unique objects: hearing apprehends sounds but not odours; the sense of smell apprehends odours but not visual images; and so forth. Knowing is also a mental faculty, and therefore there must be objects that it apprehends. These have to be unchanging, whatever they are. Plato's discovery is that there are such entities. Roughly, they are the items denoted by predicate terms in language: such words as good, white, or triangle. To say This is a triangle is to attribute a certain property, that of being a triangle, to a certain spatiotemporal object, such as a particular figure drawn on a blackboard. Plato is here distinguishing between specific triangles that can be drawn, sketched, or painted and the common property they share, that of being triangular. Objects of the former kind he calls particulars. They are always located somewhere in the space-time order, that is, in the world of appearance. But such particular things are different from the common property they share. That is, if x is a triangle, and yis a triangle, and z is a triangle, x, y, and z are particulars that share a common property, triangularity. That common property is what Plato calls a form or idea (not using this latter term in any psychological sense). Unlike particulars, forms do not exist in the space-time order. Moreover, they do not change. They are thus the objects that one must apprehend in order to acquire knowledge. Similar remarks apply, for example, to goodness, whiteness, or being to the right of. Particular things change; they come into and go out of existence. But whiteness never changes, and neither does triangularity; and, if they do not change, they are not subject to the ravages of time. In that sense, they are eternal. The use of reason for discovering unchanging forms is exercised in the dialectical method. The method is one of question and answer, designed to elicit a real definition. By a real definition is meant a set of necessary and sufficient conditions that exactly delimit a concept. One may, for example, consider the concept of being the brother of Y. This can be explained in terms of the concepts of being male and of being a sibling of Y. These concepts together lay down necessary and sufficient conditions for anything's being a brother. One who grasps these conditions understands precisely what it is to be a brother. The Republic begins with the use of the dialectical method to discover what justice is. Cephalus proposes the thesis that justice means the same as honesty in word and deed. Socrates searches for and finds a counterexample to this proposal. It is just, he points out, under some conditions, not to tell the truth or to repay debts. If one had borrowed a weapon from an insane person, who then demanded it back in order to kill an innocent person, it would be just to lie to him, stating that one no longer had the weapon. Therefore, justice cannot mean the same as honesty in word (i.e., telling the truth). By this technique of proposing one definition after another and subjecting each to possible counterexamples, Socrates attempts to find a definition that would be immune to counterexamples. To find such a definition would be to define the concept of justice, and in this way to discover the true nature of justice. In such a case one would be apprehending a form, the common feature that all just things share. Plato's search for definitions and thereby the nature of forms is a search for knowledge. But how should knowledge in general be defined? In Theaetetus Plato argues that it involves true belief. No one can know what is false. A person may mistakenly believe that he knows something, which is in fact false, but this is only thinking that one knows, not knowing. Thus, a person may confidently assert, I know that Columbus was the first European to land in North America and be unaware that other Europeans, including Erik the Red, preceded Columbus. So knowledge is at least true belief, but it must also be something more. Suppose that someone believes there will be an earthquake in September because of a dream he had in April and that there in fact is an earthquake in September, although there is no connection between the dream and the earthquake. That person has a true belief about the earthquake but not knowledge. What the person lacks is a good reason supporting his true belief. In a word, the person lacks justification for it. Thus, in Theaetetus, Plato concludes that knowledge is justified true belief. Although it is difficult to explain what justification is, most philosophers accepted the Platonic analysis of knowledge as fundamentally correct until 1963, when the American philosopher Edmund L. Gettier produced a counterexample that shook the foundations of epistemology: suppose that Kathy knows Oscar very well and that Oscar is behind her, out of sight, walking across the mall. Further, suppose that in front of her she sees walking toward her someone who looks exactly like Oscar; unbeknownst to her, it is Oscar's twin brother. Kathy forms the belief that Oscar is walking across the mall. Her belief is true, because he is walking across the mall (though she does not see him doing it). And her true belief seems to be justified, because she formed it on the same basis she would have if she had actually seen Oscar walking across the mall. Nonetheless, Kathy does not know that Oscar is walking across the mall, because the justification for her true belief is not the right kind. What her true belief lacks is an appropriate causal connection to its object. The history of epistemology Modern philosophy Faith and reason Modern philosophers as a group are usually thought to be purely secular thinkers. Nothing could be further from the truth. From the early 17th century until the middle of the 18th century, all of the great philosophers incorporated substantial religious elements into their work. Descartes, in his Meditations (1641), offered two different proofs for the existence of God, and he asserted that no one who does not believe in a cogent proof for the existence of God can have knowledge in the proper sense of the term. Benedict Spinoza began his Ethics (1677) with a proof for the existence of God, after which he expatiated on its implications for understanding all reality. And George Berkeley explained the stability of the sensible world by relying upon God's constant thought of it. Among the reasons modern philosophers are mistakenly thought to be primarily secular thinkers is that many of their epistemological principles, including some that were intended to defend religion, were later interpreted as subverting the rationality of religious belief. The role of Thomas Hobbes (15881679) and John Locke might be briefly considered in this connection. In contrast with the standard view of the Middle Ages that propositions of faith are rational, Hobbes argued that propositions of faith belong not to the intellect but to the will. To profess religious propositions is a matter of obeying the commands of a lawful authority. One need not even understand the meanings of the words professed: an obedient mouthing of the appropriate confession of faith is sufficient. In any case, the linguistic function of virtually every religious proposition is not cognitive in the sense of expressing something that is intended to represent a fact about the world but rather to give praise and honour to God. Further, in contrast to the medieval view, according to which theology is the highest science, theology is not a science at all since its propositions are not susceptible to rational dispute. In An Essay Concerning Human Understanding (1689), Locke further eroded the intellectual status of religious propositions by making them subordinate to reason in several dimensions. First, reason can dictate what the possible content of a proposition allegedly revealed by God might be; in particular, no proposition of faith can be a contradiction. Consequently, if the proposition that Jesus is both fully God and fully man is contradictory, it cannot be revealed and cannot be a matter of faith. Also, no revelation can be communicated that contains an idea not based upon sense experience. Thus, St. Paul's experience of things as eye hath not seen, nor ear heard, nor hath it entered into the heart of man to conceive, are things in which other people can have no faith. To move to another dimension in which reason takes precedence over faith, direct sense knowledge (what Locke calls intuitive knowledge) is always more certain than any alleged revelation. Thus, a person who sees that someone is soaking wet cannot have it revealed to him that the person is at that moment dry. Rational proofs, in mathematics and science, also cannot be contradicted by divine revelation. The interior angles of a rectangle equal 360, and no alleged revelation to the contrary is credible. In short, Nothing that is contrary to, and inconsistent with, the clear and self-evident dictates of reason, has a right to be urged or assented to as a matter of faith. . . . What space, then, does faith occupy within the mansion of human beliefs? According to Locke, it shares a room with probable truths, those propositions of which reason cannot be certain. There are two types: claims about observable matters of fact and claims that go beyond the discovery of our sense. Religious propositions belong to each category, as do empirical or scientific ones. That Caesar crossed the Rubicon and that Jesus walked on water belong to the first type of probable proposition. That heat is caused by the friction of imperceptibly small bodies and that angels exist are propositions that belong to the second category. While mixing religious claims with scientific ones might seem to secure a place for the former, in fact it did not. For Locke also held that whether something is a revelation or not reason must judge, and more generally that Reason must be our last judge and guide in everything. Although this maxim was intended to reconcile reason and revelationindeed, he calls reason natural revelation and revelation natural reason enlarged by a new set of discoveries communicated by Godover the course of 200 years reason repeatedly judged that alleged revelations had no scientific or intellectual standing. Although there is a strong religious element in modern thinkers, especially before the middle of the 18th century, the purely secular aspects of their thought predominate in the following discussion, because it is these that are of contemporary interest to epistemologists. Impact of modern science on epistemology Nicolaus Copernicus (14731543), a cleric, argued in On the Revolutions of the Celestial Spheres (1543) that the Earth revolves around the Sun. His theory was epistemologically shocking for at least two reasons. First, it goes directly counter to how humans experience their relation to the Sun; it is everyone's prescientific view that the Sun revolves around the Earth. If science can overthrow such a belief, then scientific reasoning seems to lead to knowledge in a way that nonscientific reasoning cannot. Indeed, the nonscientific reasoning of everyday life may seem to be a kind of superstition. Second, his theory was shocking because it contradicts the view that is presented in several books of the Bible, most importantly the explicit account in Genesis of the structure of the cosmos, according to which Earth is at the centre of creation and the Sun hangs from a celestial ceiling that holds back the waters which once flooded the Earth. If Copernicus is right, then the Bible can no longer be taken as a reliable scientific treatise. Scientific beliefs about the world, then, must be gathered in a radically new way. Many of the discoveries of Galileo Galilei (15641642) had the same two shocking consequences. His telescope seemed to reveal that unaided human vision gives false or seriously incomplete information about the nature of celestial bodies. His mathematical formulations of physical phenomena seem to indicate that most sensory information may contribute nothing to knowledge. Like his contemporary, the astronomer Johannes Kepler, he distinguished between two kinds of properties. Primary qualities, such as shape, quantity, and motion, are genuine properties of things and are knowable by mathematics. Secondary qualities, namely, odour, taste, sound, colour, warmth, or coldness, exist only in human consciousness and are not part of the objects to which they are normally attributed.