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Knowledge can be defined as awareness of facts or as practical skills, and may also refer to familiarity with objects or situations. Knowledge of facts, also called propositional knowledge, is often defined as true belief that is distinct from opinion or guesswork by virtue of justification. While there is wide agreement among philosophers that propositional knowledge is a form of true belief, many controversies in philosophy focus on justification: whether it is needed at all, how to understand it, and whether something else besides it is needed. These controversies intensified due to a series of thought experiments by Edmund Gettier and have provoked various alternative definitions. Some of them deny that justification is necessary and replace it, for example, with reliability or the manifestation of cognitive virtues. Others contend that justification is needed but formulate additional requirements, for example, that no defeaters of the belief are present or that the person would not have the belief if it was false.

Knowledge can be produced in many different ways. The most important source of empirical knowledge is perception, which refers to the usage of the senses. Many theorists also include introspection as a source of knowledge, not of external physical objects, but of one's own mental states. Other sources often discussed include memory, rational intuition, inference, and testimony. According to foundationalism, some of these sources are basic in the sense that they can justify beliefs without depending on other mental states. This claim is rejected by coherentists, who contend that a sufficient degree of coherence among all the mental states of the believer is necessary for knowledge.

Many different aspects of knowledge are investigated and it plays a role in various disciplines. It is the primary subject of the field of epistemology, which studies what we know, how we come to know it, and what it means to know something. The problem of the ''value of knowledge'' concerns the question of why knowledge is more valuable than mere true belief. Philosophical skepticism is the controversial thesis that we lack any form of knowledge or that knowledge is impossible. Formal epistemology studies, among other things, the rules governing how knowledge and related states behave and in what relations they stand to each other. Science tries to acquire knowledge using the scientific method, which is based on repeatable experimentation, observation, and measurement. Many religions hold that humans should seek knowledge and that God or the divine is the source of knowledge.

Definitions

Numerous definitions of knowledge have been suggested. Most definitions of knowledge in analytic philosophy recognize three fundamental types. "Knowledge-that", also called propositional knowledge, can be expressed using that-clauses as in "I know that Dave is at home". "Knowledge-how" (know-how) expresses practical competence, as in "she knows how to swim". Finally, "knowledge by acquaintance" refers to a familiarity with the known object based on previous direct experience. Most definitions of knowledge in analytic philosophy aim to identify the essential features of propositional knowledge. There is wide, though not universal, agreement among philosophers that knowledge involves a cognitive success or an epistemic contact with reality, and that propositional knowledge is a form of true belief.

Despite the agreement about the general characteristics of knowledge listed above, many deep disagreements remain regarding its exact definition. These disagreements relate to the goals and methods within epistemology and other fields, or to differences concerning the standards of knowledge that people intend to uphold. Some theorists focus on knowledge's most salient features in their attempt to give a practically useful definition. Others try to provide a theoretically precise definition by listing the conditions that are individually necessary and jointly sufficient. The term "analysis of knowledge" (or equivalently, "conception of knowledge" or "theory of knowledge") is often used for this approach. It can be understood in analogy to how chemists analyze a sample by seeking a list of all the chemical elements composing it. An example of this approach is characterizing knowledge as justified true belief (JTB), which is seen by many philosophers as the standard definition. Others seek a common core among diverse examples of knowledge, such as Paul Silva's "awareness first" epistemology or Barry Allen's definition of knowledge as "superlative artifactual performance".

Methodological differences concern whether researchers base their inquiry on abstract and general intuitions or on concrete and specific cases, referred to as ''methodism'' and ''particularism'', respectively. Another source of disagreement is the role of ordinary language in one's inquiry: the weight given to how the term "knowledge" is used in everyday discourse. According to Ludwig Wittgenstein, for example, there is no clear-cut definition of knowledge since it is just a cluster of concepts related through family resemblance. Different conceptions of the standards of knowledge are also responsible for various disagreements. Some epistemologists hold that knowledge demands very high requirements, like infallibility, and is therefore quite rare. Others see knowledge as a rather common phenomenon, prevalent in many everyday situations, without excessively high standards.

In analytic philosophy, knowledge is usually understood as a mental state possessed by an individual person, but the term is sometimes used to refer to a characteristic of a group of people as group knowledge, social knowledge, or collective knowledge. In a slightly different sense, it can also mean knowledge stored in documents, as in "knowledge housed in the library" or the knowledge base of an expert system.

Justified true belief

Many philosophers define knowledge as ''justified true belief'' (JTB). This definition characterizes knowledge through three essential features: as (1) a belief that is (2) true and (3) justified. In the dialogue '' Theaetetus'' by the ancient Greek philosopher Plato, Socrates pondered the distinction between knowledge and true belief but rejected the JTB definition of knowledge. The most widely accepted feature is truth: one can believe something false but one cannot know something false. A few ordinary language philosophers have raised doubts that knowledge is a form of belief based on everyday expressions like "I do not believe that; I know it". Most theorists reject this distinction and explain such expressions through ambiguities of natural language.

The main controversy surrounding the JTB definition concerns its third feature: justification. This component is often included because of the impression that some true beliefs are not forms of knowledge. Specifically, this covers cases of superstition, lucky guesses, or erroneous reasoning. The corresponding beliefs may even be true but it seems there is more to knowledge than just being right about something. The JTB definition solves this problem by identifying proper justification as the additional component needed, which is absent in the above-mentioned cases. Many philosophers have understood justification internalistically (''internalism''): a belief is justified if it is supported by another mental state of the person, such as a perceptual experience, a memory, or a second belief. This mental state has to constitute a sufficiently strong evidence or reason for the believed proposition. Some modern versions modify the JTB definition by using an ''externalist'' conception of justification instead. This means that justification depends not just on factors internal to the subject but also on external factors. They can include, for example, that the belief was produced by a reliable process or that the believed fact caused the belief.

Gettier problem and alternatives

The JTB definition came under severe criticism in the 20th century, when Edmund Gettier gave a series of counterexamples. They purport to present concrete cases of justified true beliefs that fail to constitute knowledge. The reason for their failure is usually a form of epistemic luck: the justification is not relevant to the truth. In a well-known example, there is a country road with many barn facades and only one real barn. The person driving is not aware of this, stops by a lucky coincidence in front of the real barn, and forms the belief that he is in front of a barn. It has been argued that this justified true belief does not constitute knowledge since the person wouldn't have been able to tell the difference without the fortuitous accident. So even though the belief is justified, it is a lucky coincidence that it is also true. The responses to these counterexamples have been diverse. According to some, they show that the JTB definition of knowledge is deeply flawed and that a radical reconceptualization of knowledge is necessary, often by denying justification a role. This can happen, for example, by replacing justification with reliability or by understanding knowledge as the manifestation of cognitive virtues. Other approaches include defining it in regard to the cognitive role it plays in providing reasons for doing or thinking something or seeing it as the most general factive mental state operator. Various theorists are diametrically opposed to the radical reconceptualization and either deny that Gettier cases pose problems or they try to solve them by making smaller modifications to how justification is defined. Such approaches result in a minimal modification of the JTB definition.

Between these two extremes, some philosophers have suggested various moderate departures. They agree that the JTB definition is a step in the right direction: justified true belief is a necessary condition of knowledge. However, they disagree that it is a sufficient condition. They hold instead that an additional criterion, some feature ''X'', is necessary for knowledge. For this reason, they are often referred to as ''JTB+X'' definitions of knowledge. A closely related approach speaks not of justification but of warrant and defines warrant as justification together with whatever else is necessary to arrive at knowledge. Many candidates for the fourth feature have been suggested. In this regard, knowledge may be defined as justified true belief that does not depend on any false beliefs, that there are no defeaters present, or that the person would not have the belief if it was false. Such and similar definitions are successful at avoiding many of the original Gettier cases. However, they often fall prey to newly conceived counterexamples. To avoid all possible cases, it may be necessary to find a criterion that excludes all forms of epistemic luck. It has been argued that such a criterion would set the required standards of knowledge very high: the belief has to be infallible to succeed in all cases. This would mean that very few of our beliefs amount to knowledge, if any. For example, Richard Kirkham suggests that our definition of knowledge requires that the evidence for the belief necessitates its truth. There is still very little consensus in the academic discourse as to which of the proposed modifications or reconceptualizations is correct.

Types

The English word ''knowledge'' can translate a variety of words in other languages that refer to different states. The Latin words ''cognitio'' and ''scientia'' can both be translated as "knowledge". Romance languages have two major verbs that would both be translated as "to know": for example, ''connaître'' and ''savoir'' in French or ''conocer'' and ''saber'' in Spanish. In ancient Greek, there were four such important knowledge words: '' epistēmē'' (unchanging theoretical knowledge), ''technē'' (expert technical knowledge), '' mētis'' (strategic knowledge), and '' gnōsis'' (personal intellectual knowledge). All these different types of knowledge can be considered forms of cognitive success.

Propositional knowledge

Propositional knowledge, also referred to as descriptive knowledge, is the paradigmatic type of knowledge in analytic philosophy, and various classifications are used to distinguish between its different subtypes. The distinctions between the major types are usually drawn based on the linguistic formulations used to express them. Propositional knowledge is ''propositional'' in the sense that it involves a relation to a proposition. Since propositions are often expressed through that-clauses, it is also referred to as ''knowledge-that'', as in "Akari knows that Canberra is the capital of Australia". In this case, Akari stands in the relation of knowing to the proposition "Canberra is the capital of Australia". Closely related types of knowledge are ''know-wh'', for example, ''knowing where'' the Taj Mahal is or ''knowing who'' killed J. F. Kennedy. These expressions are normally understood as types of propositional knowledge since they usually can be paraphrased using a that-clause.

Propositional knowledge is either occurrent and dispositional. This mirrors the distinction between occurrent and dispositional beliefs: to know occurrently means to entertain the corresponding representation currently, to be aware of it. "Dispositional knowledge" refers to the mere ability to do so without its execution. In this regard, a person fully immersed in a go-kart race has dispositional but not occurrent knowledge of where their home is. The reason is that they are currently occupied with something else but could easily provide this information if they stopped and focused on it.

Non-propositional knowledge

For non-propositional knowledge, no essential relation to a proposition is involved. The two most well-known forms are knowledge-how (know-how or procedural knowledge) and knowledge by acquaintance. The term "know-how" refers to some form of practical ability or skill. It can be defined as having the corresponding competence. Examples include knowing how to ride a bicycle or knowing how to play the guitar. Some of the abilities responsible for know-how may also involve certain forms of knowledge-that, as in knowing how to prove a mathematical theorem. But this is not generally the case. It is usually argued that mainly humans and maybe other higher animals possess propositional knowledge since it requires an advanced form of mind. Practical knowledge, on the other hand, is more common in the animal kingdom. In this regard, an ant knows how to walk even though it presumably lacks a mind sufficiently developed enough to stand in a relation to the corresponding proposition by representing it.

Knowledge by acquaintance refers to familiarity with an individual that results from direct experiential contact with this individual. It often, but not exclusively, concerns a relation to a person. On the linguistic level, it does not require a that-clause and can be expressed using a direct object. So when someone claims that they know Wladimir Klitschko personally, they are expressing that they had a certain kind of contact with him and not that they know a certain fact about him. This is usually understood to mean that it constitutes a relation to a concrete individual and not to a proposition. Knowledge by acquaintance plays a central role in Bertrand Russell's epistemology. He contrasts it with knowledge by description, which is a form of propositional knowledge not based on direct perceptual experience. So by watching a documentary about Wladimir Klitschko, the viewer may acquire various forms of knowledge by description about him, for example, about his nationality or his career in boxing, without acquiring knowledge by acquaintance of him. However, there is some controversy about whether it is possible to acquire knowledge by acquaintance in its pure non-propositional form. In this regard, some theorists have suggested that it might be better to understand it as one type of propositional knowledge that is only expressed in a grammatically different way.

Other distinctions

A priori and a posteriori

The distinction between a priori and a posteriori knowledge came to prominence in Immanuel Kant's philosophy and is often discussed in the academic literature. To which category a knowledge attitude belongs depends on the role of experience in its formation and justification. To know something a posteriori means to know it on the basis of experience. For example, to know that it is currently raining or that the baby is crying belongs to a posteriori knowledge since it is based on some form of experience, like visual or auditory experience. A priori knowledge, on the other hand, is possible without any experience to justify or support the known proposition. Mathematical knowledge, for example, that 2 + 2 = 4, is a paradigmatic case of a priori knowledge since no empirical investigation is necessary to confirm this fact. The distinction between a posteriori and a priori knowledge is usually equated with the distinction between empirical and non-empirical knowledge. This distinction pertains primarily to knowledge but it can also be applied to propositions or arguments. For example, an a priori proposition is a proposition that can be known independently of experience.

The prime example of the relevant experience in question is sensory experience. However, some non-sensory experiences, like memory and introspection, are often included as well. But certain conscious phenomena are excluded in this context. For example, the conscious phenomenon of a rational insight into the solution of a mathematical problem does not make the resulting knowledge a posteriori. It is sometimes pointed out that, in a trivial sense, some form of experience is required even for a priori knowledge: the experience needed to learn the language in which the claim is expressed. For a priori knowledge, this is the only form of experience required. For this reason, knowing that "all bachelors are unmarried" is considered a form of a priori knowledge since, given an understanding of the terms "bachelor" and "unmarried", no further experience is necessary to know that it is true. One difficulty for a priori knowledge is to explain how it is possible. It is usually seen as unproblematic that one can come to know things through experience but it is not clear how knowledge is possible without experience. One of the earliest solutions to this problem is due to Plato, who argues that, in the context of geometry, the soul already possesses the knowledge and just needs to recollect or remember it to access it again. A similar explanation is given by René Descartes, who holds that a priori knowledge exists as innate knowledge present in the mind of each human. A different approach is to posit a special mental faculty responsible for this type of knowledge, often referred to as ''rational insight'' or ''rational intuition''.

The distinction between a priori and a posteriori knowledge is closely related to two other distinctions: the distinction between analytic and synthetic propositions and the distinction between necessary and contingent propositions. Traditionally, it was often assumed that these distinctions coincide. On this view, a priori knowledge concerns propositions that are analytic and necessary while a posteriori knowledge is about propositions that are synthetic and contingent. However, this position is rejected by many contemporary philosophers. One reason is that these distinctions belong to different fields. The a priori–a posteriori distinction belongs to epistemology and is about how one knows things. The analytic–synthetic distinction pertains to semantics and concerns how the meanings of terms make a proposition true. The necessary–contingent distinction is metaphysical and asks, for example, whether a proposition is true in all possible worlds or just in some of them. Various discussions in the academic literature concern the question of how these distinctions overlap or fail to overlap, for example, on cases of synthetic a priori truths or of contingent a priori truths.

Self-knowledge

In philosophy, "self-knowledge" usually refers to a person's knowledge of their own sensations, thoughts, beliefs, and other mental states. Many philosophers hold that it is a special type of knowledge since it is more direct than knowledge of the external world, which is mediated through the senses. Traditionally, it was often claimed that self-knowledge is indubitable, for example, that when someone is in pain, they cannot be wrong about this fact. However, various contemporary theorists reject this position. A closely related issue is to explain how self-knowledge works. Some understand it as a form of knowledge by acquaintance while others claim that there is an inner sense that works in analogy to how the external five senses work. According to a different perspective, self-knowledge is indirect in the sense that a person has to interpret their internal and external behavior in order to learn about their mental states, similar to how one can learn about the mental states of other people by interpreting their external behavior.

In a slightly different sense, the term ''self-knowledge'' can also refer to the knowledge of the self as a persisting entity that has certain personality traits, preferences, physical attributes, relationships, goals, and social identities. This meaning is of particular interest to psychology and refers to a person's awareness of their own characteristics. Self-knowledge is closely related to self-concept, the difference being that the self-concept also includes unrealistic aspects of how a person sees themselves. In this regard, self-knowledge is often measured by comparing a person's self-assessment of their character traits with how other people assess this person's traits.

Situated knowledge

Situated knowledge is knowledge specific to a particular situation. It is closely related to practical or tacit knowledge, which is learned and applied in specific circumstances. This especially concerns certain forms of acquiring knowledge, such as trial and error or learning from experience. In this regard, situated knowledge usually lacks a more explicit structure and is not articulated in terms of universal ideas. The term is often used in feminism and postmodernism to point out that many forms of knowledge are not absolute but depend on the concrete historical, cultural, and linguistic context. Understood in this way, it is frequently used to argue against absolute or universal knowledge claims stated in the scientific discourse. Donna Haraway is a prominent defender of this position. One of her arguments is based on the idea that perception is embodied and is not a universal "gaze from nowhere". The American Psychological Association notes that some interpreters associate with this position a form of epistemological relativism because of how knowledge depends on the local conditions of the culture in which it arises.
Haraway herself criticised relativism:

Higher and lower knowledge

Many forms of eastern spirituality and religion distinguish between ''higher'' and ''lower knowledge''. They are also referred to as para vidya and apara vidya in Hinduism or the two truths doctrine in Buddhism. Lower knowledge is based on the senses and the intellect. In this regard, all forms of empirical and objective knowledge belong to this category. Most of the knowledge needed in one's everyday functioning is lower knowledge. It is about mundane or conventional things that are in tune with common sense, like that mice are smaller than elephants. It is relevant to many practical issues, like how to repair a car or how to persuade a customer. Scientific knowledge, for example, that the chemical composition of water is H₂O, is often seen as one of the most advanced forms of lower knowledge.

Higher knowledge, on the other hand, is understood as knowledge of God, the absolute, the true self, or the ultimate reality. It belongs neither to the external world of physical objects nor to the internal world of the experience of emotions and concepts. Many spiritual teachings emphasize the increased importance, or sometimes even exclusive importance, of higher knowledge in comparison to lower knowledge. This is usually based on the idea that achieving higher knowledge is one of the central steps on the spiritual path. In this regard, higher knowledge is seen as what frees the individual from ignorance, helps them realize God, or liberates them from the cycle of rebirth. This is often combined with the view that lower knowledge is in some way based on a delusion: it belongs to the realm of mere appearances or Maya, while higher knowledge manages to view the reality underlying these appearances. In the Buddhist tradition, the attainment of higher knowledge or ultimate truth is often associated with seeing the world from the perspective of sunyata, i.e. as a form of emptiness lacking inherent existence or intrinsic nature.

Sources of knowledge

Sources of knowledge are "rational capacities for knowledge" or ways how people come to know things. Various sources of knowledge are discussed in the academic literature, often in terms of the mental faculties responsible. They include perception, introspection, memory, inference, and testimony. However, not everyone agrees that all of them actually lead to knowledge. Usually, perception or observation, i.e. using one of the senses, is identified as the most important source of empirical knowledge. So knowing that the baby is sleeping constitutes observational knowledge if it was caused by a perception of the snoring baby. But this would not be the case if one learned about this fact through a telephone conversation with one's spouse. Direct realists explain observational knowledge by holding that perception constitutes a direct contact with the perceived object. Indirect realists, on the other hand, contend that this contact happens indirectly: we can only directly perceive sense data, which are then interpreted as representing external objects. This distinction is important since it affects whether the knowledge of external objects is direct or indirect and may thus have an impact on how certain the knowledge is. Introspection is often seen in analogy to perception as a source of knowledge, not of external physical objects, but of internal mental states. Traditionally, various theorists have ascribed a special epistemic status to introspection by claiming that it is infallible or that there is no introspective difference between appearance and reality. However, this claim has been contested in the contemporary discourse. Critics argue that it may be possible, for example, to mistake an unpleasant itch for a pain or to confuse the experience of a slight ellipse for the experience of a circle. Perceptual and introspective knowledge often act as a form of fundamental or basic knowledge. According to some empiricists, perceptual knowledge is the only source of basic knowledge and provides the foundation for all other knowledge.

Memory is usually identified as another source of knowledge. It differs from perception and introspection in that it is not as independent or fundamental as they are since it depends on other previous experiences. The faculty of memory retains knowledge acquired in the past and makes it accessible in the present, as when remembering a past event or a friend's phone number. It is generally considered a reliable source of knowledge, but it may deceive us at times nonetheless, either because the original experience was unreliable or because the memory degraded and does not accurately represent the original experience anymore.

Knowledge based on perception, introspection, or memory may also give rise to ''inferential knowledge'', which comes about when reasoning is applied to draw inferences from another known fact. In this regard, the perceptual knowledge of a Czech stamp on a postcard may give rise to the inferential knowledge that one's friend is visiting the Czech Republic. According to rationalists, some forms of knowledge are completely independent of observation and introspection. They are needed to explain how certain a priori beliefs, like the mathematical belief that 2 + 2 = 4, constitute knowledge. Some theorists hold that the faculty of ''pure reason'' or ''rational intuition'' is responsible in these cases since there seem to be no sensory perceptions that could justify such general and abstract knowledge. However, difficulties in providing a clear account of pure reason or rational intuition have led various empirically minded epistemologists to doubt that they constitute independent sources of knowledge. A closely related approach is to hold that this type of knowledge is ''innate''. According to Plato's theory of recollection, for example, it is accessed through a special form of remembering.

Testimony is often included as an additional source of knowledge. Unlike the other sources, it is not tied to one specific cognitive faculty. Instead, it is based on the idea that one person can come to know a fact because another person talks about this fact. Testimony can happen in numerous ways, like regular speech, a letter, the newspaper, or an online blog. The problem of testimony consists in clarifying under what circumstances and why it constitutes a source of knowledge. A popular response is that it depends on the reliability of the person pronouncing the testimony: only testimony from reliable sources can lead to knowledge.

Structure of knowledge

The expression "structure of knowledge" refers to the way in which the mental states of a person need to be related to each other for knowledge to arise. Most theorists hold that, among other things, an agent has to have good reasons for holding a belief if this belief is to amount to knowledge. So when challenged, the agent may justify their belief by referring to their reason for holding it. In many cases, this reason is itself a belief that may as well be challenged. So when the agent believes that Ford cars are cheaper than BMWs because they believe to have heard this from a reliable source, they may be challenged to justify why they believe that their source is reliable. If it turns out that their reasons are not well supported, this also affects the epistemic status of the original belief. However, whatever support they present may also be challenged. This threatens to lead to an infinite regress since the epistemic status at each step depends on the epistemic status of the previous step. Theories of the structure of knowledge offer responses for how to solve this problem.

The three most common theories are foundationalism, coherentism, and infinitism. Foundationalists and coherentists deny the existence of this infinite regress, in contrast to infinitists. According to foundationalists, some ''basic reasons'' have their epistemic status independent of other reasons and thereby constitute the endpoint of the regress. Against this view, it has been argued that the concept of "basic reason" is contradictory: there should be a reason for why some reasons are basic and others are non-basic, in which case the basic reasons would depend on another reason after all and would therefore not be basic. An additional problem consists in finding plausible candidates for basic reasons.

Coherentists and infinitists avoid these problems by denying the distinction between basic and non-basic reasons. Coherentists argue that there is only a finite number of reasons, which mutually support each other and thereby ensure each other's epistemic status. Their critics contend that this constitutes the fallacy of circular reasoning. For example, if belief ''b1'' supports belief ''b2'' and belief ''b2'' supports belief ''b1'', the agent has a reason for accepting one belief if they already have the other. However, their mutual support alone is not a good reason for newly accepting both beliefs at once. A closely related issue is that there can be various distinct sets of coherent beliefs and coherentists face the problem of explaining why we should accept one coherent set rather than another. For infinitists, in contrast to foundationalists and coherentists, there is an infinite number of reasons. This position faces the problem of explaining how human knowledge is possible at all since it seems that the human mind is limited and cannot possess an infinite number of reasons. In their traditional forms, foundationalists, coherentists and infinitists all face the Gettier problem, i.e. that having a reason or justification for a true belief is not sufficient for knowledge in cases where cognitive luck is responsible for the success.

Value of knowledge

The value of knowledge is an important topic in epistemology. Its main question is whether or why knowledge is more valuable than mere true belief. There is wide agreement that knowledge is good in some sense. For example, knowledge can help a student pass the exam or ensure that a doctor prescribes the right medicine. However, the thesis that knowledge is better than true belief is more controversial. An early discussion of this problem is found in Plato's Meno in relation to the claim that both knowledge and true belief can successfully guide action and, therefore, have apparently the same value. For example, it seems that mere true belief is as effective as knowledge when trying to find the way to Larissa. According to Plato, knowledge is better because it is more stable. A different approach is to hold that knowledge gets its additional value from justification. However, if the value in question is understood primarily as an instrumental value, it is not clear in what sense knowledge is better than mere true belief since they are usually equally useful.

The problem of the value of knowledge is often discussed in relation to reliabilism and virtue epistemology. Reliabilism can be defined as the thesis that knowledge is reliably-formed true belief. On this view, it seems difficult to explain how a reliable belief-forming process adds additional value. According to an analogy by Linda Zagzebski, a cup of coffee made by a reliable coffee machine has the same value as an equally good cup of coffee made by an unreliable coffee machine. This difficulty in solving the value problem is sometimes used as an argument against reliabilism. Virtue epistemologists have a different approach to the value problem. They see knowledge as the manifestation of cognitive virtues and can thus argue that knowledge has additional value due to its association with virtue. However, not everyone agrees that knowledge actually has additional value over true belief. A similar view is defended by Jonathan Kvanvig, who argues that the main epistemic value resides not in knowledge but in understanding, which implies grasping how one's beliefs cohere with each other.

Philosophical skepticism

Philosophical skepticism in its strongest form, also referred to as ''global skepticism'', is the thesis that we lack any form of knowledge or that knowledge is impossible. This position is quite radical and very few philosophers have explicitly defended it. However, it has been influential nonetheless, usually in a negative sense: many researchers see it as a serious challenge to any epistemological theory and often try to show how their preferred theory overcomes it. For example, it is commonly accepted that perceptual experience constitutes a source of knowledge. However, according to the dream argument, this is not the case since dreaming provides unreliable information and since the agent could be dreaming in that moment. In this case, they would be unable to distinguish actual perceptual experience from the dreaming experience. Since they may be dreaming at any time without being aware of this, it is then argued that there is no perceptual knowledge. A similar often cited thought experiment assumes that the agent is actually a brain in a vat that is just fed electrical stimuli. Such a brain would have the false impression of having a body and interacting with the external world. The basic thrust of the argument is the same: since the agent is unable to tell the difference, they do not know that they have a body responsible for reliable perceptions.

One issue revealed through these thought experiments is the problem of underdetermination: that the evidence available is not sufficient to make a rational decision between competing theories. And if two contrary hypotheses explain the appearances equally well then the agent is not justified in believing one of those hypotheses rather than the other. Based on this premise, the general skeptic just has to argue that this is true for all our knowledge, that there is always an alternative and very different explanation. Another skeptic argument is based on the idea that human cognition is fallible and therefore lacks absolute certainty. More specific arguments target particular theories of knowledge, such as foundationalism or coherentism, and try to show that their concept of knowledge is deeply flawed. An important argument against global skepticism is that it seems to contradict itself: the claim that there is no knowledge appears to constitute a knowledge-claim itself. Other responses come from common sense philosophy and reject global skepticism based on the fact that it contradicts common sense. It is then argued against skepticism by seeing common sense as more reliable than the abstract reasoning cited in favor of skepticism.

Certain less radical forms of skepticism deny that knowledge exists within a specific area or discipline, sometimes referred to as ''local'' or ''selective skepticism''. It is often motivated by the idea that certain phenomena do not accurately represent their subject matter. They may thus lead to false impressions concerning its nature. External world skeptics hold that we can only know about our own sensory impressions and experiences but not about the external world. This is based on the idea that beliefs about the external world are mediated through the senses. The senses are faulty at times and may thus show things that are not really there. This problem is avoided on the level of sensory impressions, which are given to the experiencer directly without an intermediary. In this sense, the person may be wrong about seeing a red Ferrari in the street (it might have been a Maserati or a mere light reflection) but they cannot be wrong about having a sensory impression of seeing a patch of red color. The inverse path is taken by some materialists, who accept the existence of the external physical world but deny the existence of the internal realm of mind and consciousness based on the difficulty of explaining how the two realms can exist together. Other forms of local skepticism accept scientific knowledge but deny the possibility of moral knowledge, for example, because there is no reliable way to empirically measure whether a moral claim is true or false.

The issue of the definition and standards of knowledge is central to the question of whether skepticism in its different forms is true. If very high standards are used, for example, that knowledge implies infallibility, then skepticism becomes more plausible. In this case, the skeptic only has to show that no belief is absolutely certain; that while the actual belief is true, it could have been false. However, the more these standards are weakened to how the term is used in everyday language, the less plausible skepticism becomes.

In various disciplines

Formal epistemology

Formal epistemology studies knowledge using formal tools, such as mathematics and logic. An important issue in this field concerns the epistemic principles of knowledge. They are rules governing how knowledge and related states behave and in what relations they stand to each other. The transparency principle, also referred to as the ''luminosity of knowledge'', is an often discussed principle. It states that knowing something implies the second-order knowledge that one knows it. So if Heike knows that today is Monday, then she also knows that she knows that today is Monday. According to the ''conjunction principle'', having two justified beliefs in two separate propositions implies that the agent is also justified in believing in the conjunction of these two propositions. The ''closure principle'' states that if the agent has a justified belief in one proposition and this proposition entails another proposition, then the agent is also justified in believing this other proposition. The ''evidence transfer principle'' applies this idea to evidence: if, in the case above, a certain piece of evidence justifies the first belief then it also justifies the second belief.

Science

The development of the scientific method has made a significant contribution to how knowledge of the physical world and its phenomena is acquired. To be termed scientific, a method of inquiry must be based on gathering observable and measurable evidence subject to specific principles of reasoning and experimentation. The scientific method consists of the collection of data through observation and experimentation, and the formulation and testing of hypotheses. Science and the nature of scientific knowledge have also become the subject of philosophy. As science itself has developed, scientific knowledge now includes a broader usage in the soft sciences such as the social sciences. Sir Francis Bacon was critical of the historical development of the scientific method; his works established and popularized an inductive methodology for scientific inquiry. His aphorism, "knowledge is power", is found in the Meditations Sacrae (1597).

Religion

Knowledge plays a central role in many religions. Knowledge claims about the existence of God or religious doctrines about how each one should live their lives are found in almost every culture. However, such knowledge claims are often controversial and are commonly rejected by religious skeptics and atheists. The epistemology of religion is the field of inquiry that investigates whether belief in God and in other religious doctrines is rational and amounts to knowledge. It is different from other forms of epistemology because of its unique subject matter. One important view in this field is evidentialism. It states that belief in religious doctrines is justified if it is supported by sufficient evidence. Suggested examples of evidence for religious doctrines include religious experiences such as direct contact with the divine or inner testimony as when hearing God's voice. However, evidentialists often reject that belief in religious doctrines amount to knowledge based on the claim that we lack sufficient evidence. A famous saying in this regard is due to Bertrand Russell. When asked how he would justify his lack of belief in God when facing His judgment after death, he replied "Not enough evidence, God! Not enough evidence."

However, religious teachings about the existence and nature of God are not always understood as knowledge claims by their defenders and some explicitly state that the proper attitude towards such doctrines is not knowledge but faith. This is often combined with the assumption that these doctrines are true but cannot be fully understood by reason or verified through rational inquiry. For this reason, it is claimed that one should accept them even though they do not amount to knowledge. Such a view is reflected in a famous saying by Immanuel Kant where he claims that he "had to deny knowledge in order to make room for faith."

Distinct religions often differ from each other concerning the doctrines they proclaim as well as their understanding of the role of knowledge in religious practice. Knowledge plays a central role in Abrahamic religions. In Jewish tradition, knowledge (Hebrew: דעת ''da'ath'') is considered one of the most valuable traits a person can acquire. Observant Jews recite three times a day in the Amidah "You favor people with knowledge and teach mortals understanding. Favor us with your knowledge, understanding, and wisdom. Blessed are you, Adonai, who favors people with knowledge." The Tanakh

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