Epistemology (/ɪˌpɪstɪˈmɒlədʒi/ (listen); from Greek ἐπιστήμη, epistēmē, meaning 'knowledge', and -logy) is the branch of philosophy concerned with the theory of knowledge.

Epistemology is the study of the nature of knowledge, justification, and the rationality of belief. Much debate in epistemology centers on four areas: (1) the philosophical analysis of the nature of knowledge and how it relates to such concepts as truth, belief, and justification,^[1][2] (2) various problems of skepticism, (3) the sources and scope of knowledge and justified belief, and (4) the criteria for knowledge and justification. Epistemology addresses such questions as: "What makes justified beliefs justified?",^[3] "What does it mean to say that we know something?",^[4] and fundamentally "How do we know that we know?"^[5]

The word epistemology is derived from the ancient Greek epistēmē meaning "knowledge" and the suffix -logy, meaning "logical discourse" (derived from the Greek word logos meaning "discourse"). It is analogue to the German Wissenschaftslehre (literally, theory of science) which was introduced by philosophers Johann Fichte and Bernard Bolzano in the late 18th century. The word first appeared in English in 1847 as a translation of the German in New York's Eclectic Magazine review of a philosophical novel by German author Jean Paul:^[6][7]

It was properly introduced in the philosophical literature by Scottish philosopher James Frederick Ferrier in his Institutes of Metaphysics (1854):^[8]

French philosophers then gave the term épistémologie a narrower meaning as philosophy of science. E.g., Émile Meyerson opened his Identity and Reality, written in 1908, with the remark that the word 'is becoming current' as equivalent to 'the philosophy of the sciences.'^[9]

The idea of epistemology predates the word. John Locke describes his Essay Concerning Human Understanding (1689) as an inquiry "into the original, certainty, and extent of human knowledge, together with the grounds and degrees of belief, opinion, and assent".^[10] According to Brett Warren, the character Epistemon in King James VI of Scotland's Daemonologie (1591) "was meant to be a personification of a philosophical concept currently known as 'epistemology': the investigation into the differences of a justified belief versus its opinion."^[11]

In mathematics, it is known that 2 + 2 = 4, but there is also knowing how to add two numbers, and knowing a person (e.g., knowing other persons,^[12] or knowing oneself), place (e.g., one's hometown), thing (e.g., cars), or activity (e.g., addition). Some philosophers think there is an important distinction between "knowing that" (know a concept), "knowing how" (understand an operation), and "acquaintance-knowledge" (know by relation), with epistemology being primarily concerned with the first of these.^[13]

While these distinctions are not explicit in English, they are defined explicitly in other languages (N.B. some languages related to English have been said to retain these verbs, e.g. Scots: wit and ken). In French, Portuguese, Spanish, German and Dutch 'to know (a person)' is translated using connaître, conhecer, conocer and kennen (both German and Dutch) respectively, whereas 'to know (how to do something)' is translated using savoir, saber (both Portuguese and Spanish), wissen, and weten. Modern Greek has the verbs γνωρίζω (gnorízo) and ξέρω (kséro). Italian has the verbs conoscere and sapere and the nouns for 'knowledge' are conoscenza and sapienza. German has the verbs wissen and kennen; the former implies knowing a fact, the latter knowing in the sense of being acquainted with and having a working knowledge of; there is also a noun derived from kennen, namely Erkennen, which has been said to imply knowledge in the form of recognition or acknowledgment.^[14] The verb itself implies a process: you have to go from one state to another, from a state of "not-erkennen" to a state of true erkennen. This verb seems the most appropriate in terms of describing the "episteme" in one of the modern European languages, hence the German name "Erkenntnistheorie". The theoretical interpretation and significance of these linguistic issues remains controversial.

In his paper On Denoting and his later book Problems of Philosophy Bertrand Russell stressed the distinction between "knowledge by description" and "knowledge by acquaintance". Gilbert Ryle is also credited with stressing the distinction between knowing how and knowing that in The Concept of Mind. In Personal Knowledge, Michael Polanyi argues for the epistemological relevance of knowledge how and knowledge that; using the example of the act of balance involved in riding a bicycle, he suggests that the theoretical knowledge of the physics involved in maintaining a state of balance cannot substitute for the practical knowledge of how to ride, and that it is important to understand how both are established and grounded. This position is essentially Ryle's, who argued that a failure to acknowledge the distinction between knowledge that and knowledge how leads to infinite regress.

In recent times, epistemologists including Sosa, Greco, Kvanvig,^[15] Zagzebski and Duncan Pritchard have argued that epistemology should evaluate people's "properties" (i.e., intellectual virtues) and not just the properties of propositions or of propositional mental attitudes.

In common speech, a "statement of belief" is typically an expression of faith or trust in a person, power or other entity—while it includes such traditional views, epistemology is also concerned with what we believe. This includes 'the' truth, and everything else we accept as 'true' for ourselves from a cognitive point of view.

Whether someone's belief is true is not a prerequisite for (its) belief. On the other hand, if something is actually known, then it categorically cannot be false. For example, if a person believes that a bridge is safe enough to support her, and attempts to cross it, but the bridge then collapses under her weight, it could be said that she believed that the bridge was safe but that her belief was mistaken. It would not be accurate to say that she knew that the bridge was safe, because plainly it was not. By contrast, if the bridge actually supported her weight, then the person might say that she had believed the bridge was safe, whereas now, after proving it to herself (by crossing it), she knows it was safe.

Epistemologists argue over whether belief is the proper truth-bearer. Some would rather describe knowledge as a system of justified true propositions, and others as a system of justified true sentences. Plato, in his Gorgias, argues that belief is the most commonly invoked truth-bearer.^[16]

In the Theaetetus, Socrates considers a number of theories as to what knowledge is, the last being that knowledge is true belief "with an account" (meaning explained or defined in some way). According to the theory that knowledge is justified true belief, to know that a given proposition is true, one must not only believe the relevant true proposition, but also have a good reason for doing so. One implication of this would be that no one would gain knowledge just by believing something that happened to be true. For example, an ill person with no medical training, but with a generally optimistic attitude, might believe that he will recover from his illness quickly. Nevertheless, even if this belief turned out to be true, the patient would not have known that he would get well since his belief lacked justification.

The definition of knowledge as justified true belief was widely accepted until the 1960s. At this time, a paper written by the American philosopher Edmund Gettier provoked major widespread discussion. (See theories of justification for other views on the idea.)

Edmund Gettier is best known for his 1963 paper entitled Is Justified True Belief Knowledge?, which called into question the theory of knowledge that had been dominant among philosophers for thousands of years.^[17] This in turn called into question the actual value of philosophy if such an obvious and easy counterexample to a major theory could exist without anyone noticing it for thousands of years. In a few pages, Gettier argued that there are situations in which one's belief may be justified and true, yet fail to count as knowledge. That is, Gettier contended that while justified belief in a true proposition is necessary for that proposition to be known, it is not sufficient. As in the diagram, a true proposition can be believed by an individual (purple region) but still not fall within the "knowledge" category (yellow region).

According to Gettier, there are certain circumstances in which one does not have knowledge, even when all of the above conditions are met. Gettier proposed two thought experiments, which have become known as Gettier cases, as counterexamples to the classical account of knowledge. One of the cases involves two men, Smith and Jones, who are awaiting the results of their applications for the same job. Each man has ten coins in his pocket. Smith has excellent reasons to believe that Jones will get the job and, furthermore, knows that Jones has ten coins in his pocket (he recently counted them). From this Smith infers, "The man who will get the job has ten coins in his pocket." However, Smith is unaware that he also has ten coins in his own pocket. Furthermore, Smith, not Jones, is going to get the job. While Smith has strong evidence to believe that Jones will get the job, he is wrong. Smith has a justified true belief that the man who will get the job has ten coins in his pocket; however, according to Gettier, Smith does not know that the man who will get the job has ten coins in his pocket, because Smith's belief is "...true by virtue of the number of coins in Jones's pocket, while Smith does not know how many coins are in Smith's pocket, and bases his belief...on a count of the coins in Jones's pocket, whom he falsely believes to be the man who will get the job." (see^[17] p. 122.) These cases fail to be knowledge because the subject's belief is justified, but only happens to be true by virtue of luck. In other words, he made the correct choice (believing that the man who will get the job has ten coins in his pocket) for the wrong reasons. This example is similar to those often given when discussing belief and truth—wherein a person's belief of what will happen can coincidentally be correct without the actual knowledge to base it on.

The responses to Gettier have been varied. Usually, they have involved substantial attempts to provide a definition of knowledge different from the classical one, either by recasting knowledge as justified true belief with some additional fourth condition, or proposing a completely new set of conditions, disregarding the classical ones entirely.

In one response to Gettier, the American philosopher Richard Kirkham has argued that the only definition of knowledge that could ever be immune to all counterexamples is the infallibilist one.^[18] To qualify as an item of knowledge, goes the theory, a belief must not only be true and justified, the justification of the belief must necessitate its truth. In other words, the justification for the belief must be infallible.

Yet another possible candidate for the fourth condition of knowledge is indefeasibility. Defeasibility theory maintains that there should be no overriding or defeating truths for the reasons that justify one's belief. For example, suppose that person S believes he saw Tom Grabit steal a book from the library and uses this to justify the claim that Tom Grabit stole a book from the library. A possible defeater or overriding proposition for such a claim could be a true proposition like, "Tom Grabit's identical twin Sam is currently in the same town as Tom." When no defeaters of one's justification exist, a subject would be epistemologically justified.

The Indian philosopher B.K. Matilal has drawn on the Navya-Nyāya fallibilism tradition to respond to the Gettier problem. Nyaya theory distinguishes between know p and know that one knows p—these are different events, with different causal conditions. The second level is a sort of implicit inference that usually follows immediately the episode of knowing p (knowledge simpliciter). The Gettier case is examined by referring to a view of Gangesha Upadhyaya (late 12th century), who takes any true belief to be knowledge; thus a true belief acquired through a wrong route may just be regarded as knowledge simpliciter on this view. The question of justification arises only at the second level, when one considers the knowledge-hood of the acquired belief. Initially, there is lack of uncertainty, so it becomes a true belief. But at the very next moment, when the hearer is about to embark upon the venture of knowing whether he knows p, doubts may arise. "If, in some Gettier-like cases, I am wrong in my inference about the knowledge-hood of the given occurrent belief (for the evidence may be pseudo-evidence), then I am mistaken about the truth of my belief—and this is in accordance with Nyaya fallibilism: not all knowledge-claims can be sustained."^[19]

Reliabilism has been a significant line of response to the Gettier problem among philosophers, originating with work by Alvin Goldman in the 1960s. According to reliabilism, a belief is justified (or otherwise supported in such a way as to count towards knowledge) only if it is produced by processes that typically yield a sufficiently high ratio of true to false beliefs. In other words, this theory states that a true belief counts as knowledge only if it is produced by a reliable belief-forming process. Examples of reliable processes include: standard perceptual processes, remembering, good reasoning, and introspection.^[20]

Reliabilism has been challenged by Gettier cases. Another argument that challenges reliabilism, like the Gettier cases (although it was not presented in the same short article as the Gettier cases), is the case of Henry and the barn façades. In the thought experiment, a man, Henry, is driving along and sees a number of buildings that resemble barns. Based on his perception of one of these, he concludes that he has just seen barns. While he has seen one, and the perception he based his belief that the one he saw was of a real barn, all the other barn-like buildings he saw were façades. Theoretically, Henry does not know that he has seen a barn, despite both his belief that he has seen one being true and his belief being formed on the basis of a reliable process (i.e. his vision), since he only acquired his true belief by accident.^[21]

Robert Nozick has offered the following definition of knowledge: S knows that P if and only if:

Nozick argues that the third of these conditions serves to address cases of the sort described by Gettier. Nozick further claims this condition addresses a case of the sort described by D.M. Armstrong:^[23] A father believes his daughter is innocent of committing a particular crime, both because of faith in his baby girl and (now) because he has seen presented in the courtroom a conclusive demonstration of his daughter's innocence. His belief via the method of the courtroom satisfies the four subjunctive conditions, but his faith-based belief does not. If his daughter were guilty, he would still believe her innocence, on the basis of faith in his daughter; this would violate the third condition.

The British philosopher Simon Blackburn has criticized this formulation by suggesting that we do not want to accept as knowledge beliefs, which, while they "track the truth" (as Nozick's account requires), are not held for appropriate reasons. He says that "we do not want to award the title of knowing something to someone who is only meeting the conditions through a defect, flaw, or failure, compared with someone else who is not meeting the conditions."^[24] In addition to this, externalist accounts of knowledge, such as Nozick's, are often forced to reject closure in cases where it is intuitively valid.

Timothy Williamson has advanced a theory of knowledge according to which knowledge is not justified true belief plus some extra condition(s), but primary. In his book Knowledge and its Limits, Williamson argues that the concept of knowledge cannot be broken down into a set of other concepts through analysis—instead, it is sui generis. Thus, according to Williamson, justification, truth, and belief are necessary but not sufficient for knowledge.

Alvin Goldman writes in his "Causal Theory of Knowing" that knowledge requires a causal link between the truth of a proposition and the belief in that proposition.

A central debate about the nature of justification is a debate between epistemological externalists on the one hand, and epistemological internalists on the other.

Externalists hold that factors deemed "external", meaning outside of the psychological states of those who gain knowledge, can be conditions of justification. For example, an externalist response to the Gettier problem is to say that for a justified true belief to count as knowledge, there must be a link or dependency between the belief and the state of the external world. Usually this is understood to be a causal link. Such causation, to the extent that it is "outside" the mind, would count as an external, knowledge-yielding condition. Internalists, on the other hand, assert that all knowledge-yielding conditions are within the psychological states of those who gain knowledge.

Though unfamiliar with the internalist/externalist debate himself, many point to René Descartes as an early example of the internalist path to justification. He wrote that, because the only method by which we perceive the external world is through our senses, and that, because the senses are not infallible, we should not consider our concept of knowledge infallible. The only way to find anything that could be described as "indubitably true", he advocates, would be to see things "clearly and distinctly".^[25] He argued that if there is an omnipotent, good being who made the world, then it's reasonable to believe that people are made with the ability to know. However, this does not mean that man's ability to know is perfect. God gave man the ability to know but not with omniscience. Descartes said that man must use his capacities for knowledge correctly and carefully through methodological doubt.^[26]

The dictum "Cogito ergo sum" (I think, therefore I am) is also commonly associated with Descartes' theory. In his own methodological doubt—doubting everything he previously knew so he could start from a blank slate—the first thing that he could not logically bring himself to doubt was his own existence: "I do not exist" would be a contradiction in terms. The act of saying that one does not exist assumes that someone must be making the statement in the first place. Descartes could doubt his senses, his body, and the world around him—but he could not deny his own existence, because he was able to doubt and must exist to manifest that doubt. Even if some "evil genius" were deceiving him, he would have to exist to be deceived. This one sure point provided him with what he called his Archimedean point, in order to further develop his foundation for knowledge. Simply put, Descartes' epistemological justification depended on his indubitable belief in his own existence and his clear and distinct knowledge of God.^[27]

We generally assume that knowledge is more valuable than mere true belief. If so, what is the explanation? A formulation of the value problem in epistemology first occurs in Plato's Meno. Socrates points out to Meno that a man who knew the way to Larissa could lead others there correctly. But so, too, could a man who had true beliefs about how to get there, even if he had not gone there or had any knowledge of Larissa. Socrates says that it seems that both knowledge and true opinion can guide action. Meno then wonders why knowledge is valued more than true belief and why knowledge and true belief are different. Socrates responds that knowledge is more valuable than mere true belief because it is tethered or justified. Justification, or working out the reason for a true belief, locks down true belief.^[28]

The problem is to identify what (if anything) makes knowledge more valuable than mere true belief, or that makes knowledge more valuable than a mere minimal conjunction of its components, such as justification, safety, sensitivity, statistical likelihood, and anti-Gettier conditions, on a particular analysis of knowledge that conceives of knowledge as divided into components (to which knowledge-first epistemological theories, which posit knowledge as fundamental, are notable exceptions).^[29] The value problem re-emerged in the philosophical literature on epistemology in the twenty-first century following the rise of virtue epistemology in the 1980s, partly because of the obvious link to the concept of value in ethics.^[30]

The value problem has been presented as an argument against epistemic reliabilism by philosophers including Linda Zagzebski, Wayne Riggs and Richard Swinburne. Zagzebski analogizes the value of knowledge to the value of espresso produced by an espresso maker: "The liquid in this cup is not improved by the fact that it comes from a reliable espresso maker. If the espresso tastes good, it makes no difference if it comes from an unreliable machine."^[31] For Zagzebski, the value of knowledge deflates to the value of mere true belief. She assumes that reliability in itself has no value or disvalue, but Goldman and Olsson disagree. They point out that Zagzebski's conclusion rests on the assumption of veritism: all that matters is the acquisition of true belief.^[32] To the contrary, they argue that a reliable process for acquiring a true belief adds value to the mere true belief by making it more likely that future beliefs of a similar kind will be true. By analogy, having a reliable espresso maker that produced a good cup of espresso would be more valuable than having an unreliable one that luckily produced a good cup because the reliable one would more likely produce good future cups compared to the unreliable one.

The value problem is important to assessing the adequacy of theories of knowledge that conceive of knowledge as consisting of true belief and other components. According to Kvanvig, an adequate account of knowledge should resist counterexamples and allow an explanation of the value of knowledge over mere true belief. Should a theory of knowledge fail to do so, it would prove inadequate.^[33]

One of the more influential responses to the problem is that knowledge is not particularly valuable and is not what ought to be the main focus of epistemology. Instead, epistemologists ought to focus on other mental states, such as understanding.^[34] Advocates of virtue epistemology have argued that the value of knowledge comes from an internal relationship between the knower and the mental state of believing.^[29]

The nature of this distinction has been disputed by various philosophers; however, the terms may be roughly defined as follows:

A priori knowledge is a way of gaining knowledge without the need of experience. In Bruce Russell's article "A Priori Justification and Knowledge"^[35] he says that it is "knowledge based on a priori justification," (1) which relies on intuition and the nature of these intuitions. A priori knowledge is often contrasted with posteriori knowledge, which is knowledge gained by experience. A way to look at the difference between the two is through an example. Bruce Russell gives two propositions in which the reader decides which one he believes more. Option A: All crows are birds. Option B: All crows are black. If you believe option A, then you are a priori justified in believing it because you don't have to see a crow to know it's a bird. If you believe in option B, then you are posteriori justified to believe it because you have seen many crows therefore knowing they are black. He goes on to say that it doesn't matter if the statement is true or not, only that if you believe in one or the other that matters.

The idea of a priori knowledge is that it is based on intuition or rational insights. Laurence BonJour says in his article "The Structure of Empirical Knowledge",^[36] that a "rational insight is an immediate, non-inferential grasp, apprehension or 'seeing' that some proposition is necessarily true." (3) Going back to the crow example, by Laurence BonJour's definition the reason you would believe in option A is because you have an immediate knowledge that a crow is a bird, without ever experiencing one.

Evolutionary psychology takes a novel approach to the problem. It says that there is an innate predisposition for certain types of learning. "Only small parts of the brain resemble a tabula rasa; this is true even for human beings. The remainder is more like an exposed negative waiting to be dipped into a developer fluid".^[37]

Immanuel Kant, in his Critique of Pure Reason, drew a distinction between "analytic" and "synthetic" propositions. He contended that some propositions are such that we can know they are true just by understanding their meaning. For example, consider, "My father's brother is my uncle." We can know it is true solely by virtue of our understanding in what its terms mean. Philosophers call such propositions analytic". Synthetic propositions, on the other hand, have distinct subjects and predicates. An example would be, "My father's brother has black hair." Kant stated that all mathematical and scientific statements are analytic priori propositions because they are necessarily true but our knowledge about the attributes of the mathematical or physical subjects we can only get by logical inference.

The American philosopher Willard Van Orman Quine, in his Two Dogmas of Empiricism, famously challenged the distinction, arguing that the two have a blurry boundary. Some contemporary philosophers have offered more sustainable accounts of the distinction.^[38]

Science is viewed as a refined, formalized, systematic, or institutionalized form of the pursuit and acquisition of empirical knowledge. As such, the philosophy of science may be viewed variously as an application or foundation of the philosophy of knowledge acquisition.

The regress problem is the problem of providing a complete logical foundation for human knowledge. The traditional way of supporting a rational argument is to appeal to other rational arguments, typically using chains of reason and rules of logic. A classic example that goes back to Aristotle is deducing that Socrates is mortal. We have a logical rule that says All humans are mortal and an assertion that Socrates is human and we deduce that Socrates is mortal. In this example how do we know that Socrates is human? Presumably we apply other rules such as: All born from human females are human. Which then leaves open the question how do we know that all born from humans are human? This is the regress problem: how can we eventually terminate a logical argument with some statement(s) that do not require further justification but can still be considered rational and justified? As John Pollock stated:

The apparent impossibility of completing an infinite chain of reasoning is thought by some to support skepticism. It is also the impetus for Descartes' famous dictum: I think, therefore I am. Descartes was looking for some logical statement that could be true without appeal to other statements.

Many epistemologists studying justification have attempted to argue for various types of chains of reasoning that can escape the regress problem.

Foundationalists respond to the regress problem by asserting that certain "foundations" or "basic beliefs" support other beliefs but do not themselves require justification from other beliefs. These beliefs might be justified because they are self-evident, infallible, or derive from reliable cognitive mechanisms. Perception, memory, and a priori intuition are often considered possible examples of basic beliefs.

The chief criticism of foundationalism is that if a belief is not supported by other beliefs, accepting it may be arbitrary or unjustified.^[40]

Another response to the regress problem is coherentism, which is the rejection of the assumption that the regress proceeds according to a pattern of linear justification. To avoid the charge of circularity, coherentists hold that an individual belief is justified circularly by the way it fits together (coheres) with the rest of the belief system of which it is a part. This theory has the advantage of avoiding the infinite regress without claiming special, possibly arbitrary status for some particular class of beliefs. Yet, since a system can be coherent while also being wrong, coherentists face the difficulty of ensuring that the whole system corresponds to reality. Additionally, most logicians agree that any argument that is circular is, at best, only trivially valid. That is, to be illuminating, arguments must operate with information from multiple premises, not simply conclude by reiterating a premise.

Warburton writes in Thinking from A to Z, "Circular arguments are not invalid; in other words, from a logical point of view there is nothing intrinsically wrong with them. However, they are, when viciously circular, spectacularly uninformative." (Warburton 1996)

A position known as foundherentism, advanced by Susan Haack, is meant to unify foundationalism and coherentism. One component of this theory is what is called the "analogy of the crossword puzzle." Whereas, for example, infinitists regard the regress of reasons as "shaped" like a single line, Susan Haack has argued that it is more like a crossword puzzle, with multiple lines mutually supporting each other.^[41]

An alternative resolution to the regress problem is known as "infinitism". Infinitists take the infinite series to be merely potential, in the sense that an individual may have indefinitely many reasons available to them, without having consciously thought through all of these reasons when the need arises. This position is motivated in part by the desire to avoid what is seen as the arbitrariness and circularity of its chief competitors, foundationalism and coherentism.

The historical study of philosophical epistemology is the historical study of efforts to gain philosophical understanding or knowledge of the nature and scope of human knowledge.^[42] Since efforts to get that kind of understanding have a history, the questions philosophical epistemology asks today about human knowledge are not necessarily the same as they once were.^[42] But that does not mean that philosophical epistemology is itself a historical subject, or that it pursues only or even primarily historical understanding.^[42]

In philosophy, empiricism is generally a theory of knowledge focusing on the role of experience, especially experience based on perceptual observations by the senses.^[43] Certain forms exempt disciplines such as mathematics and logic from these requirements.^[44]

There are many variants of empiricism: positivism, realism and common sense being among the most commonly expounded. But central to all empiricist epistemologies is the notion of the epistemologically privileged status of sense data.

Many idealists believe that knowledge is primarily (at least in some areas) acquired by a priori processes or is innate—for example, in the form of concepts not derived from experience. The relevant theoretical processes often go by the name "intuition".^[45] The relevant theoretical concepts may purportedly be part of the structure of the human mind (as in Kant's theory of transcendental idealism), or they may be said to exist independently of the mind (as in Plato's theory of Forms).

By contrast with empiricism and idealism, which centres around the epistemologically privileged status of sense data (empirical) and the primacy of reason (theoretical) respectively, modern rationalism adds a third 'system of thinking', (as Gaston Bachelard has termed these areas) and holds that all three are of equal importance: The empirical, the theoretical and the abstract. For Bachelard, rationalism makes equal reference to all three systems of thinking.

Constructivism is a view in philosophy according to which all "knowledge is a compilation of human-made constructions",^[46] "not the neutral discovery of an objective truth".^[47] Whereas objectivism is concerned with the "object of our knowledge", constructivism emphasizes "how we construct knowledge".^[48] Constructivism proposes new definitions for knowledge and truth that form a new paradigm, based on inter-subjectivity instead of the classical objectivity, and on viability instead of truth. Piagetian constructivism, however, believes in objectivity—constructs can be validated through experimentation. The constructivist point of view is pragmatic;^[49] as Vico said: "The norm of the truth is to have made it."

Pragmatism is an empiricist epistemology formulated by Charles Sanders Peirce, William James, and John Dewey, which understands truth as that which is practically applicable in the world. Peirce formulates the maxim: 'Consider what effects, that might conceivably have practical bearings, we conceive the object of our conception to have. Then, our conception of these effects is the whole of our conception of the object.'^[50] This suggests that we are to analyse ideas and objects in the world for their practical value. This is in contrast to any correspondence theory of truth that holds that what is true is what corresponds to an external reality. William James suggests that through a pragmatist epistemology 'Theories thus become instruments, not answers to enigmas in which we can rest.'^[51] A more contemporary understanding of pragmatism was developed by the philosopher Richard Rorty who proposed that values were historically contingent and dependent upon their utility within a given historical period.^[52]

Closely related to Pragmatism, naturalized epistemology considers the evolutionary role of knowledge for agents living and evolving in the world.^[53] It de-emphasizes the questions around justification and truth, and instead asks, empirically, what beliefs should agents hold in order to survive. It suggests a more empirical approach to the subject as a whole—leaving behind philosophical definitions and consistency arguments, and instead using psychological methods to study and understand how knowledge actually forms and is used in the natural world. As such it does not attempt to answer the analytic questions of traditional epistemology but replace them with new empirical ones.

Indian philosophical schools such as the Hindu Nyaya, and Carvaka, and later, the Jain and Buddhist philosophical schools, developed an epistemological tradition termed "pramana" independently of the Western philosophical tradition. Pramana can be translated as "instrument of knowledge" and refers to various means or sources of knowledge that Indian philosophers held to be reliable. Each school of Indian philosophy had their own theories about which pramanas were valid means to knowledge and which were unreliable (and why).^[54] A Vedic text, Taittirīya Āraṇyaka (c. 9th–6th centuries BCE), lists "four means of attaining correct knowledge": smṛti ("tradition" or "scripture"), pratyakṣa ("perception"), aitihya ("communication by one who is expert", or "tradition), and anumāna ("reasoning" or "inference").^[55][56]

In the Indian traditions, the most widely discussed pramanas are: Pratyakṣa (perception), Anumāṇa (inference), Upamāṇa (comparison and analogy), Arthāpatti (postulation, derivation from circumstances), Anupalabdi (non-perception, negative/cognitive proof) and Śabda (word, testimony of past or present reliable experts). While the Nyaya school (beginning with the Nyāya Sūtras of Gotama, between 6th-century BCE and 2nd-century CE^[57][58]) were a proponent of realism and supported four pramanas (perception, inference, comparison/analogy and testimony), the Buddhist epistemologists (Dignaga and Dharmakirti) generally accepted only perception and inference.

The theory of knowledge of the Buddha in the early Buddhist texts has been interpreted as a form of pragmatism as well as a form of correspondence theory.^[59] Likewise, the Buddhist philosopher Dharmakirti has been interpreted both as holding a form of pragmatism or correspondence theory for his view that what is true is what has effective power (arthakriya).^[60][61] The Buddhist Madhyamika school's theory of emptiness (shunyata) meanwhile has been interpreted as a form of philosophical skepticism.^[62]

The main contribution to epistemology by the Jains has been their theory of "many sided-ness" or "multi-perspectivism" (Anekantavada), which says that since the world is multifaceted, any single viewpoint is limited (naya – a partial standpoint).^[63] This has been interpreted as a kind of pluralism or perspectivism.^[64][65] According to Jain epistemology, none of the pramanas gives absolute or perfect knowledge since they are each limited points of view.

The Carvaka school of materialists only accepted the pramana of perception and hence were one of the first empiricists.^[66] There was also another school of philosophical skepticism, the Ajñana.

Skepticism is a position that questions the validity of some or all of human knowledge. Skepticism does not refer to any one specific school of philosophy, rather it is a thread that runs through many philosophical discussions of epistemology. The first well known Greek skeptic was Socrates who claimed that his only knowledge was that he knew nothing with certainty. In Indian philosophy, Sanjaya Belatthiputta was a famous skeptic and the Buddhist Madhyamika school has been seen as taking up a form of skepticism. Descartes' most famous inquiry into mind and body also began as an exercise in skepticism. Descartes began by questioning the validity of all knowledge and looking for some fact that was irrefutable. In so doing, he came to his famous dictum: I think, therefore I am.

Foundationalism and the other responses to the regress problem are essentially defenses against skepticism. Similarly, the pragmatism of William James can be viewed as a coherentist defense against skepticism. James discarded conventional philosophical views of truth and defined truth as based on how well a concept works in a specific context, rather than objective rational criteria. The philosophy of Logical Positivism and the work of philosophers such as Kuhn and Popper can be viewed as skepticism applied to what can truly be considered scientific knowledge.^[67]

Skepticism questions whether knowledge is possible at all. Skeptics argue that belief in something does not justify whether or not it is necessarily true.^[68] Characterizing knowledge as strong or weak is dependent on a person's viewpoint and their characterization of knowledge.^[68] Much of our knowledge on epistemology is derived from, in particular, rational and philosophical skepticism.^[69]

The evil demon skepticism described by Descartes (previously known from Plato's cave; with an updated version from sci-fi literature describing it as the brain in a vat) supposes that our sensors have been placed under the control of some external power such as a demon, mad scientist, etc. As such, everything we see is a fake, and we can never know anything about the 'real' world inhabited by the demon or mad scientist. Even if these external powers do not exist, we still must depend on only the information provided by our senses and can therefore make no definite statement about anything beyond that information.

Skeptics oppose what they sometimes call dogmatic foundationalism, which states that there must be some basic positions that are self-justified or beyond justification without reference to others. (One example of such foundationalism may be found in Spinoza's Ethics.) The skeptical response to this can take several approaches. First, claiming that "basic positions" must exist amounts to the logical fallacy of argument from ignorance combined with the slippery slope.

Among other arguments, skeptics used Agrippa's trilemma, named after Agrippa the Skeptic, to claim certain belief could not be achieved. Foundationalists have used the same trilemma argument as justification for demanding the validity of basic beliefs.

This skeptical approach is rarely taken to its pyrrhonean extreme by most practitioners. Several modifications have arisen over the years, including the following Peter Suber, "Classical Skepticism" ^[121] :

Fictionalism would not claim to have knowledge but adheres to conclusions on some criterion such as utility, aesthetics, or other personal criteria without claiming that any conclusion is actually "true".

Philosophical fideism (as opposed to religious Fideism) would assert the truth of some propositions, but does so without asserting certainty.

Some forms of pragmatism would accept utility as a provisional guide to truth but not necessarily a universal decision-maker.

There are two different categories of epistemological skepticism, which can be referred to as "mitigated" and "unmitigated" skepticism. The two forms are contrasting but are still true forms of skepticism. Mitigated skepticism does not accept "strong" or "strict" knowledge claims but does, however, approve specific weaker ones. These weaker claims can be assigned the title of "virtual knowledge", but only to justified beliefs. Unmitigated skepticism rejects both claims of virtual knowledge and strong knowledge.^[68] Characterizing knowledge as strong, weak, virtual or genuine can be determined differently depending on a person's viewpoint as well as their characterization of knowledge.^[68]