Knowledge

Before knowledge became a matter for analytic treatment at the hands of Plato and Aristotle, it had already been set out by the tragedians as a problem of human life. The clearest instance is the Oedipus the King of Sophocles, where the movement of the play from beginning to end is the movement from ignorance to recognition, and that movement is the hero's ruin. The oracle has declared what Oedipus will do and what he already has done; the action of the drama is the account of how he comes to see it. At each step he insists on pursuing the inquiry, over the warnings first of Tiresias and then of Jocasta, and each step brings him nearer to the knowledge that will destroy him.

What Sophocles places before us is not a theory of knowledge but a condition in which knowledge is desired and feared at once. Oedipus is king because he has solved a riddle, and he assumes that questions rightly put will always yield their answers. That confidence, which is the ground of his authority and the assumption of his later pursuit, turns out to be the very instrument of his undoing. The knowledge he obtains is not a knowledge of causes in the scientific sense but a recognition, or anagnorisis, in which the facts already present to others become present at last to himself. The distinction between knowing and acknowledging, which later writers will treat in other forms, is here set out in a shape that admits of no softening.

The play thus raises questions that occupy the whole subsequent tradition. Whether ignorance can itself be a kind of happiness, whether the soul is made for a truth which it cannot bear, whether there is a kind of knowledge that only suffering will give: these questions are implicit in the Sophoclean treatment and reappear in many forms. The connection between such recognition and the workings of Fate is discussed under that head; the bearing of tragic discovery on the emotions of pity and fear is considered under Emotion and under Poetry.

Plato in the Republic proposes a different relation between knowledge and life, one in which the ascent from appearance to reality issues not in ruin but in the vision of the Good. His account of the philosopher's journey out of the Cave can be read, on at least one point, as an answer to what the tragedians had dramatized: where Sophoclean knowledge destroys, Platonic knowledge is meant to save. Whether the consolation will hold, and whether the philosophic life can do what the tragic hero could not, is a question the tradition returns to in Ecclesiastes, in Pascal, and in the moderns who place a limit on what reason may attain.

Plato draws a sharp distinction between knowledge and opinion, a distinction that determines the whole course of the subsequent discussion. Knowledge, in Plato's view, is of the Forms: eternal, unchanging realities accessible to reason alone. Opinion concerns the shifting world of sensible particulars. In the Meno, the slave boy who recovers geometric truths under Socratic questioning illustrates the doctrine that knowledge is recollection: the soul, having known the Forms before its embodiment, recovers what it once possessed when prompted by the right questions. The difference between knowledge and right opinion, as Socrates insists, is not a matter of truth alone, since right opinion can also be true, but of the grounds on which the truth is held.

In the Republic, Plato elaborates this distinction through a series of images. The Form of the Good, like the sun, is the source both of the being and the knowability of all intelligible objects. The Divided Line represents four stages of apprehension, from the lowest (images and shadows) to the highest (direct intellectual vision of the Forms). The allegory of the Cave describes the philosopher's ascent from the world of appearances to the light of reality. Knowledge, in the fullest sense, requires this ascent from becoming to being, from the visible to the intelligible.

This conception commits Plato to a view of knowledge as inseparable from certitude. To speak of "false knowledge" or "uncertain knowledge" appears to him self-contradictory. Knowledge, properly so called, admits of neither falsity nor doubt. The consequence is that the whole domain of sensible experience, subject as it is to change and variability, falls outside the sphere of genuine knowledge and belongs to the realm of opinion. The bearing of this distinction on the question of education is treated under the idea of Education; its bearing on the nature of truth is considered under Truth.

Plato's separation of knowledge from opinion, and his identification of knowledge with the apprehension of eternal and immutable objects, establishes the terms within which the theory of knowledge is debated by all subsequent writers. Aristotle, while preserving much of Plato's account of knowledge as concerned with the universal and the necessary, will reject the separation of the Forms from sensible things and insist that knowledge must begin with what the senses deliver.

Aristotle agrees with Plato that knowledge, in the strict sense, is of the universal and the necessary, but he differs fundamentally in locating the forms of things within sensible particulars rather than in a separate realm. To know, for Aristotle, is to grasp the causes of a thing: material, formal, efficient, and final. The opening sentence of the Metaphysics, that all men by nature desire to know, sets the framework for an account of knowledge that begins with sense perception and ascends, through memory, experience, and induction, to universal principles.

In the Posterior Analytics, Aristotle distinguishes knowing that something is the case from scientific knowledge of why it is the case. Scientific demonstration proceeds from first principles, grasped by intuitive reason, through syllogistic reasoning, to conclusions that could not be otherwise. The first principles of any science cannot themselves be demonstrated, for demonstration requires premises more certain than the conclusion. These principles are apprehended by a faculty Aristotle calls nous, which perceives the universal in the particular after repeated experience. The relation between induction and demonstration, and the question of how first principles are secured, is treated more fully under the ideas of Induction and Principle.

Aristotle's account yields a classification of the sciences into theoretic, productive, and practical, each distinguished by its object, its manner of reasoning, and its end. Theoretic science, which includes physics, mathematics, and theology, seeks knowledge for its own sake. Practical science, such as ethics and politics, aims at the guidance of action. Productive science is concerned with the making of things. This tripartite division, though stated in different language by later writers such as Bacon and Kant, remains the fundamental framework for distinguishing kinds of knowledge.

Aristotle's causal-demonstrative conception of knowledge provides the terms in which the question is discussed throughout the mediaeval period, most notably by Aquinas. The question he leaves open, concerning how first principles are derived from experience with sufficient certainty to serve as the foundations of demonstration, is the point at which Bacon and the modern empiricists will press their challenge.

Augustine adopts the Platonic view that genuine knowledge is of eternal and unchanging truths, but he locates these truths not in a separate realm of Forms but in the mind of God. The means by which finite human minds apprehend such truths is divine illumination: God is the inner light of the intellect, as the sun is the outer light of the eye. By this light the soul is enabled to see intelligible truths that it could not discover through the senses or through the unaided operations of reason.

In On the Teacher, Augustine argues that words and signs cannot, strictly speaking, convey knowledge from one mind to another. When a pupil grasps what a teacher says, it is not because the words themselves transmit understanding but because the pupil, consulting the interior light, recognizes the truth for himself. The true teacher, on this account, is not the human speaker but Christ, the interior teacher who illumines every mind. The relation of signs to knowledge is treated more fully under the idea of Language, but Augustine's position bears directly on the theory of knowledge by establishing that understanding is not transferred from outside but awakened from within.

Augustine's epistemology is thus interior in character. The path to truth runs through self-examination, not because the self is its own source of truth but because the self, when rightly ordered, is capable of receiving the divine light in which eternal truths are seen. The connection between knowledge and love, which Plato suggests in the Symposium, receives in Augustine a theological development: to know God is inseparable from loving God, and the highest knowledge is not merely intellectual but involves the whole orientation of the soul toward its proper object.

Augustine's doctrine of divine illumination provides the framework within which the Christian tradition discusses knowledge until Aquinas. The question it leaves open is how illumination operates through the ordinary processes of sense experience and abstraction, a question to which Aquinas will give a different answer by incorporating the Aristotelian account of how knowledge arises from the cooperation of senses and intellect.

Aquinas attributes to man the kind of knowledge appropriate to his station in the hierarchy of beings. Man is superior to the brutes because he has a faculty of reason in addition to the faculties of sense and imagination which he shares with them. But man is inferior to the angels and to God because, being corporeal, his intellect cannot function independently of his bodily senses. Accordingly, all human knowledge is both sensitive and intellectual, never mere sense-perception as with the brutes nor pure intellectual intuition as with the angels.

The essential process of knowing, for Aquinas, begins with the senses. The intellect does not possess innate ideas; it comes to know through the cooperation of sense and reason. The active intellect works upon the sensory images produced by experience, abstracting from them the intelligible form, which is then received by the passive intellect. "Nothing is in the intellect that was not first in the senses." This analysis denies innate ideas, denies man's power to apprehend ideas intuitively apart from sense, and affirms that knowledge is primarily of real existence rather than of relations between ideas alone.

Yet this empirical account of the origin of knowledge does not exclude a theological dimension. The natural light of reason by which we abstract and judge is itself a created participation in the divine intellect. Moreover, Aquinas distinguishes sharply between knowledge gained through man's own natural efforts and knowledge received through divine revelation. In addition to all knowledge acquired by the natural exercise of his faculties, man may be elevated by the supernatural gift of wisdom, a faith surpassing reason. The relation between natural and revealed knowledge is treated more fully under the idea of Theology.

Aquinas's synthesis of the Aristotelian and Augustinian traditions provides the most systematic mediaeval account of knowledge. The points on which later writers disagree with it, particularly concerning innate ideas, the limits of sense experience, and the role of the intellect in structuring what it knows, define the central issues that Descartes, Locke, and Kant will address in their own theories of knowledge.

Montaigne's "Apology for Raymond Sebond" is the most sustained skeptical meditation in the tradition of the great books prior to Hume. Surveying the disagreements of philosophers, the variability of custom, the unreliability of the senses, and the instability of human judgment, Montaigne concludes that certainty in matters of natural knowledge is beyond our reach. "What do I know?" is the question he inscribes as his motto, and his examination of human understanding is conducted in the spirit of that question.

Montaigne's skepticism is of the moderate rather than the extreme variety. He does not deny that we have beliefs and working opinions, nor does he hold that all propositions are equally uncertain. In the realm of action, he admits the need for judgments of probability. But he denies that these amount to the kind of certain knowledge that the Scholastic tradition claimed to possess. Reason, he observes, is a variable instrument: it serves whatever cause employs it, justifying opposite conclusions with equal facility. The contradictory of everything has been asserted or argued by someone. "The persuasion of certainty is a certain testimony of folly and extreme uncertainty." The bearing of this skepticism on the question of custom is treated more fully under the idea of Custom and Convention.

Montaigne's position, however, is not one of despair. He exempts matters of religious faith from his skepticism, conceding that what cannot be known by natural reason may yet be received through revelation. And his response to the limits of knowledge is not inaction but a turn toward self-examination: if we cannot know the world with certainty, we can at least observe and describe honestly the operations of our own minds in all their variability and contradiction. This turn inward makes his Essays one of the founding documents of the modern concern with the knowing subject.

Montaigne's skepticism provides the challenge to which Descartes directly responds. His question, what can we actually claim to know with certainty, sets the terms for the modern attempt to establish knowledge on foundations that skeptical doubt cannot dissolve. Hume will later describe a similar form of "mitigated skepticism" as both durable and useful, though he arrives at it by a different path.

Descartes proposes a method of universal doubt as the means of establishing knowledge on firm foundations. In the Meditations on First Philosophy, he resolves to doubt everything that can be doubted: the testimony of the senses, which sometimes deceive; the apparent certainty of waking experience, which might be indistinguishable from dream; and even the truths of mathematics, which an omnipotent deceiver might cause him to misapprehend. What survives this universal doubt is the one thing that cannot be doubted without being affirmed: the existence of the doubting self. "I think, therefore I am."

From this starting point, Descartes undertakes to rebuild the edifice of knowledge. Examining the idea of a perfect being that he finds within his own mind, he argues that God must exist and, being perfectly good, would not permit systematic deception concerning what the mind perceives clearly and distinctly. The criterion of knowledge thus becomes clarity and distinctness of perception, guaranteed by the veracity of God. Mathematics, whose objects are grasped with such clarity, is therefore trustworthy. The physical world, known through clear and distinct ideas of extension, is knowable to the degree that it can be so grasped. The question of what constitutes a clear and distinct idea, and whether the mind can reliably distinguish such ideas from those it merely believes to be clear, is a difficulty that Locke and Hume will press.

Descartes's method marks a departure from the ancients. Where Aristotle and Aquinas treat the examination of knowledge as a reflection on knowledge already possessed, Descartes makes the inquiry into the possibility of knowledge a necessary preparation for all further inquiry. The consideration of knowing is put before any attempt to know. This precedence of epistemology over all other subjects, what Kant will later call the critical approach, begins with Descartes, though it receives its fullest development in Kant's own work.

Descartes's method of doubt and his criterion of clear and distinct ideas constitute the foundation of modern rationalism. Locke will challenge the doctrine of innate ideas on which parts of this system depend, and Hume will question whether the guarantee of divine veracity is not itself a proposition requiring the very certainty it is meant to establish.

Locke sets himself the program of inquiring "into the original, certainty, and extent of human knowledge, together with the grounds and degrees of belief, opinion, and assent." He begins with a systematic attack on the doctrine of innate ideas: if any principles were innate, they would be universally assented to, but children and the unlettered manifestly lack them. The mind at birth is white paper, void of all characters. All the materials of reason and knowledge are furnished by experience.

Locke distinguishes two sources of experience: sensation, which provides ideas of external objects, and reflection, which gives the mind awareness of its own operations. From these two sources arise all simple ideas, which the understanding then combines, compares, and abstracts into complex ideas. Knowledge itself he defines as the perception of the agreement or disagreement of two ideas. This definition confines knowledge within relatively narrow limits, for certainty can be had only where such agreement or disagreement is clearly perceived. Beyond this lies the sphere of judgment and probability, which, though it falls short of knowledge properly so called, is sufficient for the practical guidance of life.

Locke's inquiry into the extent of knowledge leads him to distinguish between those objects about which certainty is attainable and those about which we can have only probable opinion. He recognizes that our knowledge of the external world is mediated by ideas that may not resemble their causes in all respects. His distinction between primary qualities, which belong to bodies themselves, and secondary qualities, which are powers to produce sensations in us, bears on the question, treated more fully under the idea of Sense, of how far our ideas correspond to the realities they represent. The practical counsel Locke draws from his examination of the understanding is that when we know our own strength we shall the better know what to undertake with hopes of success.

Locke's empiricism establishes the framework within which Hume and Kant conduct their inquiries. Hume will press the question that Locke's definition of knowledge raises: if knowledge is only of the relations between ideas, and if all ideas derive from experience, then our confidence in the existence of an external world and in the necessity of causal connections may rest on grounds less secure than Locke supposed.

Hume sharpens the empiricist principle into a criterion for distinguishing knowledge from mere speculation: every idea must be traced to a prior impression of sense or inner feeling, and any concept that cannot be so traced is without content. This principle, rigorously applied, restricts the scope of genuine knowledge more severely than Locke had envisioned. The only objects with respect to which demonstration is possible are quantity and number; mathematics alone possesses the certitude of knowledge. All other matters are "matters of fact and existence," and these, Hume maintains, are incapable of demonstration.

The most consequential application of this principle concerns causation. We observe one event regularly followed by another, but we never observe any necessary connection between them. All we perceive is constant conjunction. Our conviction that the future will resemble the past, and that one event must produce another, is not given in experience but is a habit of mind, a customary expectation generated by the repeated experience of conjunction. "All inferences from experience, therefore, are effects of custom, not of reasoning." The bearing of this analysis on the nature of science and the status of induction is treated more fully under the ideas of Cause and Induction.

Hume describes his own position as a "mitigated skepticism" that avoids the extremes of maintaining either that nothing is knowable or that everything is equally knowable. Extreme skepticism, or Pyrrhonism, he considers impractical: "the great subverter of Pyrrhonism is action, and employment, and the occupations of life." But a mitigated skepticism, which consists in becoming sensible of the limitations of human understanding and confining inquiry to subjects adapted to its capacity, he regards as both durable and useful. Probabilities are the best that experimental reasoning about matters of fact can achieve, and if probability is characteristic of opinion rather than knowledge, then we can have nothing better than opinion concerning real existences.

Hume's analysis provides the challenge that Kant describes as rousing him from his "dogmatic slumber." If the foundations of scientific knowledge rest not on rational demonstration but on psychological habit, the question becomes whether some third alternative, between dogmatism and skepticism, can be found. Kant's critical philosophy is the attempt to find it.

Kant takes up the question of knowledge in a spirit different from both the dogmatists and the skeptics. He uses the word "critique" to signify his intention to examine reason's own powers before permitting it to make claims about objects beyond experience. He does not object to what he calls the dogmatical procedure of reason in the development of science, but insists that it must be preceded by reason's self-criticism. For Kant, dogmatism and skepticism are the opposite excesses which only a critical method can avoid.

His proposal, which he compares to the Copernican revolution in astronomy, is that instead of assuming our knowledge must conform to objects, we should consider whether objects, as we experience them, must conform to the conditions under which knowledge is possible. Space and time are the a priori forms of sensibility: nothing can be experienced except as spatially and temporally arranged. Concepts such as substance, causation, and unity are a priori categories of the understanding, the conditions under which any experience can be coherent at all. Though all knowledge begins with experience, it does not follow that all knowledge arises out of experience; the forms and categories that the mind contributes to experience are not derived from it.

The consequence of this analysis is a strict limitation on what we can know. Knowledge extends only to the world of phenomena, that is, to objects as they appear under the conditions of human experience. Things as they are in themselves, which Kant calls noumena, are not objects of knowledge. Metaphysics, insofar as it claims to know God, the soul, or freedom as objects of theoretical reason, exceeds the boundaries of possible experience and therefore the boundaries of knowledge. The questions that metaphysics raises are not answered by theoretical reason but by practical reason, as treated under the ideas of God and Duty.

Kant's critical philosophy represents the most systematic attempt to determine the scope and limits of human knowledge. Hegel will contest its central claim by arguing that the distinction between appearances and things in themselves is itself a distinction made by thought, and that what thought distinguishes it cannot coherently place beyond its own reach.

Hegel contests Kant's central limitation on knowledge. If the thing in itself is placed beyond the reach of thought, Hegel argues, then the very distinction between appearances and things in themselves is itself a distinction made by thought. What thought distinguishes it cannot coherently exclude from its own domain. The consequence, for Hegel, is that the separation between knowing and being on which Kant's system rests cannot be maintained. Reality is rational, and the rational is real; there is no residue of unknowable being standing outside the reach of the concept.

Knowledge, on Hegel's account, is not a fixed possession but a developmental process. The Phenomenology of Spirit traces the journey of consciousness through a series of stages, each of which reveals its own inadequacies and gives rise to a more comprehensive form of knowing. Sense-certainty gives way to perception, perception to understanding, understanding to self-consciousness, and self-consciousness, through the experience of recognition and conflict, to reason. Each stage does not simply replace the last but takes it up into a higher unity. The truth, for Hegel, is "the whole": the complete developmental process, not any single moment within it.

This conception introduces a historical dimension into the theory of knowledge that was absent from the tradition before Hegel. What counts as knowledge is not determined by the isolated individual examining his own faculties, as in Descartes or Locke, but is achieved by a community of knowers over time, through contradiction, struggle, and reconciliation. The relation between knowledge and history is treated more fully under the idea of History, but it bears directly on epistemology by suggesting that the forms of knowing available to any age are conditioned by the stage of development that the human spirit has reached.

Hegel's identification of knowledge with the self-development of spirit represents the most ambitious claim about the scope of human knowing in the tradition. William James will approach the question from a different direction, asking not whether thought can in principle comprehend all of reality, but whether the test of any particular claim to knowledge is the difference it makes in the conduct of experience.

William James brings to the discussion of knowledge a conception that departs from both the rationalist and the empiricist traditions. Knowledge, he insists, involves a "thoroughgoing dualism" between mind and thing known; "neither gets out of itself or into the other, neither in any way is the other." But the question of what makes a belief count as knowledge is, for James, a question about consequences rather than about correspondence to a fixed reality. The true is "the name of whatever proves itself to be good in the way of belief, and good, too, for definite, assignable reasons."

This pragmatic criterion treats truth not as a static property of propositions but as something that happens to ideas in the course of experience. An idea becomes true, is "made true by events," insofar as it leads successfully through experience, enables prediction, coheres with other well-tested beliefs, and serves the purposes of the knower. James does not deny that there is a reality to which thought refers, but he maintains that the only test of whether thought has succeeded in grasping reality is the practical difference the thought makes. If two hypotheses have identical consequences in experience, the dispute between them is, on James's view, merely verbal. The relation between truth and utility is treated more fully under the idea of Truth.

James's position represents an attempt to mediate between the extremes that have defined the discussion of knowledge since its beginning. Against the skeptic, he maintains that knowledge is possible and that beliefs can be tested. Against the dogmatist, he denies that any belief is immune to revision in the light of further experience. His mitigated position has something in common with Hume's advocacy of a useful and durable skepticism, though it differs in grounding its conclusions not in the limits of human faculties but in the character of truth as an evolving relation between the knower and the experienced world.

James's pragmatism closes the classical discussion of knowledge by reframing its central question. Rather than asking what knowledge is, as the tradition from Plato onward has done, he asks what difference it makes to have it. Whether this reframing answers the traditional question or changes the subject is itself a matter on which the tradition remains divided.