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Many people think that they have a lot of knowledge. They also believe that other people sometimes know what they claim to know. But what is knowledge anyway? And how do we come to have it? Is it important that we have knowledge? If so, why?
The branch of philosophy that attempts to answer these and related questions is known as ‘epistemology’ or ‘theory of knowledge.’
1. What is Knowledge?
One historically popular definition of ‘knowledge’ is the ‘JTB’ theory of knowledge: knowledge is justified, true belief. Most philosophers think that a belief must be true in order to count as knowledge. Suppose that Smith is framed for a crime, and the evidence against Smith is overwhelming. But Smith is innocent. Does the jury know that Smith committed the crime? No, because knowledge requires truth; the jury believed it, but they didn’t know it.
Also, most philosophers think that the belief must be justified. We don’t normally consider a lucky guess to be knowledge. (If you flip a coin but don’t look at the result yet, and just find yourself believing that it’s Heads, and in fact it is Heads, we don’t think you knew that it was Heads.) Yet there is substantial debate about what justification is.
2. Analysis of Justification
What would be required to transform the lucky guess that the coin came up Heads into knowledge? One standard answer is ‘justification.’
Intuitively, you would have to acquire good reason to believe that the coin came up Heads: for example, you look at the coin. In general, theories of justification either say that the factors that justify your belief must be available within your own first-person, conscious experience, or that they don’t need to be. We call the former theories ‘internalist’ and the latter ‘externalist.’
For example, some internalists are evidentialists: roughly, they say that your justification depends on the evidence you have, for example the awareness of the image of a president’s head on a coin. And some externalists are reliabilists: roughly, they say that whether you are justified depends on whether your belief was formed by a reliable belief-forming mechanism, for example, the physical process of vision. For evidentialists, the evidence you have seems “internal” to your first-person awareness: it’s what you’re aware of. And whether the process that formed your belief is reliable seems “external” to your first-person awareness: you might not even have any beliefs about photons and how they work.
3. Sources of Knowledge and Evidence
We’ve thought about what it means to have justified beliefs. But when our beliefs are in fact justified, what is the source or explanation of that justification?
Surely we get some justification from empirical observation: using our five senses, the tools of science, and introspection (looking inside your own mind, for example learning that you believe that 2+2=4). This justification is ‘a posteriori’ or ‘empirical,’ for example, looking at the coin-toss result.
Maybe there is some knowledge that we acquire through intuition, definitions, or some other putative non-empirical evidence. We call this ‘a priori’ knowledge or justification: justified but independently of any particular empirical observation or awareness of the evidence or justification required. I seem to know, just by thinking about it, that there can be no coins that are both square and circular; I don’t need to consult scientists.
Roughly speaking, rationalists (about justification) believe that we have important, valuable a priori knowledge about the world, knowledge that’s not merely about the meanings of our words or concepts. Empiricists, in contrast, believe that all of our important knowledge of the world is empirical.
Beyond this, feminist epistemologists have argued that one’s knowledge, and whether one is taken seriously as a knower, can depend on one’s particular social position, including one’s gender. To the traditional list of sources of knowledge, we might add one’s standpoint and situation, and also add emotion.
4. The Structure of Knowledge, Justification, and Inference
Normally, we think we can infer new beliefs from our existing beliefs: we believe some propositions, and on the basis of those beliefs, conclude that some other propositions are true.
If someone asks you to justify a belief, you’re likely to cite other beliefs you have. I know that Antarctica exists because I believe that maps are generally trustworthy and that maps claim that Antarctica exists. But can this regress of inference go on forever?
One popular position is foundationalism. Foundationalists say that the regress stops at justified beliefs and, since the regress has stopped, those beliefs aren’t justified by any inference from any other belief.
Another popular position is coherentism, according to which one’s beliefs are justified by being part of a web or network of coherent beliefs.
We think a set of inferences can give us justified beliefs. But it’s possible to acquire false or unjustified beliefs, which wouldn’t be knowledge after all.
In general, skeptics deny that we have some item of knowledge. There are skeptics about whether we have:
- knowledge of the external world, i.e., the world beyond the contents of our conscious experiences;
- moral knowledge;
- religious knowledge;
- knowledge from memory or knowledge of the past;
- knowledge from induction;
- scientific knowledge; and
- knowledge of other minds.
And some philosophers have endorsed global skepticism, according to which we have no knowledge or no justified beliefs at all.
6. Other Issues
Epistemology is a broad field. Other issues include:
- the nature of perception;
- the nature of understanding;
- whether we should trust our peers when they disagree with us;
- the relationships between epistemology, trust, and justice;
- how different reasons for belief (not just epistemic reasons) should affect us or determine whether we have knowledge;
- what value there is, if any, in having knowledge, especially in contrast to mere true belief;
- whether ‘knowledge’ means the same thing in all contexts.
- whether knowledge requires certainty; 
- whether philosophy should be continuous with, or even replaced by, science.
And many more.
Some philosophers believe that epistemology is “first philosophy,” fundamental to the rest of philosophy and inquiry, since all areas of study and learning seek understanding, justified beliefs, and knowledge. If they are correct, an understanding of at least the basics of epistemology is important for all thinking people.
 In constructing this overview, I roughly follow online encyclopedias such as in Steup 2019 and Truncellito 2019 and textbooks such as BonJour 2002, Huemer 2002, and Steup and Sosa 2005. ‘Epistemology’ comes from Greek roots meaning the study of, or discourse about, knowledge.
 Here, we are talking about what’s sometimes called ‘propositional’ knowledge or ‘knowledge-that.’ But there are other mental facts about you that may count as your having knowledge. Someone might know how to ride a bicycle, or know what it’s like to taste lemon. These aren’t obviously propositional knowledge, but they are natural enough ways of using the term ‘knowledge.’
 Gettier (1963) attributes this conception or something very much like it to Plato (Theaetetus 20 I; Meno 98), Chisholm (1957: 16), and Ayer (1956). BonJour (2002: 27) attributes a similar conception to Descartes. Yet as Gettier argues, there may be examples of justified, true beliefs that don’t count as ‘knowledge.’ Suppose that your computer has an error and its internal clock is stuck at 11:49 am. It stays this way all morning and you don’t notice. Then you happen to wonder what time it is, look at your computer’s clock, and see ‘11:49 am.’ Suppose, however, that the time actually just happens to be 11:49 am right when you look at the clock. Your belief that it is 11:49 am might be justified (because you have no reason yet to doubt your computer’s clock), and true (because you happened to check the clock right at 11:49 am), but many would say it doesn’t count as ‘knowledge.’ Therefore, perhaps there is some fourth property that a belief must have to count as knowledge. For more, see Zagzebski 1994 and Chapman 2014 (“The Gettier Problem” in 1000-Word Philosophy).
 Sometimes a person might say, ‘I just knew I wasn’t going to get that job–but I did!’ But arguably, that use of ‘knew’ is intended to emphasize the person’s certainty, not to say that they really knew something false. But see Hazlett 2010 which argues that knowledge doesn’t require truth.
 And see n. 4.
 Although the term of art for ‘whatever has to be added to true belief to create knowledge’ is ‘warrant.’ Cf. Plantinga 1993. Perhaps Gettier (1963) showed that warrant isn’t merely justification.
 See e.g. BonJour 2004: 5 on what epistemic justification is. Philosophers have described justification as occurring when a belief is likely to be true, or when a person who wants to have true beliefs ought to believe that belief, or when one has good reason to believe the belief, or when the belief is best-supported by the evidence. This question is actually fairly complex, however, because some (e.g. possibly Foley 1987: ch. 1; but see Kelly 2003 for critique) believe that epistemic justification is just a species of prudential or instrumental justification.
 I generally follow Pappas 2019; refer there for more information. In more detail: an internalist might say (as ‘accessibilists’ say) that justification depends on what you are aware of from the first-person perspective, or (as ‘mentalists’ say) on the current content of your mental states, or (as ‘evidentialists’ say) on the evidence available to you. Externalists might say (as ‘reliabilists’ say) that justification depends on one’s belief’s having been formed by a reliable belief-forming mechanism, or (as ‘proper functionalists’ say) by the proper functioning of your cognitive apparatus. Most generally, evidentialism, accessibilism, mentalism, and deontology are usually thought of as ‘internalist’ theories of epistemic justification, since they say your justification depends on factors internal to your mind or first-person awareness. And reliabilism, proper-functionalism, and virtue epistemology are normally thought of as ‘externalist’ theories of epistemic justification, since whether a belief was reliably formed, for example, may depend on factors you have no first-person access to. See e.g. Fumerton 1995 and BonJour and Sosa 2003, as well as Pappas 2019. One might also take the position that internalists and externalists are simply describing two different phenomena, both philosophically important, that are sometimes both referred to as ‘justification; cf. BonJour 2002: 233-7 and Pasnau 2013: 1008 ff. For more about specific internalist and externalist theories, see Conee and Feldman 2004 for evidentialism; Chisholm 1977: 17 for accessibilism; Conee and Feldman 2001 for mentalism; Alston 1989: 115-52 for deontology; Goldman 1979 for reliabilism; Plantinga 1993: 41 ff. and Bergmann 2006: ch. 5 for proper-functionalism; and Turri et al. 2019 for an introduction to virtue epistemology.
 One more note: There could be justified, false beliefs. For example, someone might do a really great job of framing you for a crime you didn’t commit, and so people would be justified in believing that you committed the crime, yet you really are innocent and so their justified belief is false. Such beliefs are part of the standard presentation of Gettier (1963) cases. Of course, a few philosophers have questioned whether justified or warranted false beliefs are possible (Sturgeon 1993; Merricks 1995).
 See for example Steup 2019: § 4.2. I assume that introspection is fundamentally empirical, at least in the sense necessary to make the a priori-empirical distinction most illuminating (BonJour 1998: 7).
 But not normally independent of any empirical observation at all. For example, we might need empirical observation to acquire the relevant concept in the first place. See e.g. BonJour 1998: 9.
 Perhaps moral knowledge, if it exists, is also a priori (cf. Huemer 2005). It’s difficult to imagine seeing, with my eyes, that stealing is wrong.
 Some famous rationalists include Plato, Descartes, and Leibniz; some famous empiricists include Locke, Berkeley, and Hume. Yet these categorizations aren’t 100% neat; for example, Locke and Berkeley are normally counted among the empiricists, but they may have made some limited room for a priori knowledge as well. See Markie 2019: § 1.2. Also, strictly speaking, when I talk about these different types of sentences, what I’m referring to is the distinction between analytic sentences (very roughly, sentences that are true by definition, e.g. ‘all red squares are red’) and synthetic sentences (sentences that are not merely true by definition, e.g. ‘all red squares are somewhere on Earth’), such that rationalists believe in the synthetic a priori and empiricists don’t. For a general introduction to this debate, see Markie 2019.
 See Anderson 2020 for an overview.
 Jaggar 1988.
 Infinitists (e.g. Klein 1998) say that the regress goes back forever, but that our beliefs can still be justified.
 One of the most-important historical presentations of foundationalism is in Descartes (cf. Newman 2019: § 2.1). More-recent defenders include Pryor (2000) and Huemer (2001). Foundationalists face the challenge of explaining what could justify a belief other than an inference from some other belief. One answer that has received much attention recently is to appeal to some principle of conservatism; see e.g. Huemer 2001. Perhaps, for example, if it appears to you as if something is true, that appearance is enough to justify the belief.
 To say that these beliefs are coherent might mean, for example, that they are all logically consistent and mutually supporting. Thus for coherentists, in some sense, the regress of inference “circles back” on itself. Influential coherentists include Lewis (1946); Quine and Ullian (1970) and the early BonJour (1985). A more recent example is Poston (2014). Coherentists face their own challenges, for example, why there needs to be any “input” from outside one’s own mind (cf. BonJour 1985: ch. 6) in order to have a set of justified beliefs. Why isn’t mere coherence with the rest of one’s beliefs enough?
 See e.g. Descartes 1984 : 13.
 Sinnott-Armstrong 2020.
 Forrest 2020.
 Cf. Russell 2019 : 71-84.
 Hume 1896 [1739-40]: bk. I, part III, § VI.
 See e.g. Chakravartty 2019 for an overview of scientific realism and anti-realism.
 Mill 1872: 243; Duddington 1919; Ayer 1953.
 Comesaña and Klein 2019: § 5.
 See e.g. Berkeley 1904 ; Austin 1962; and Lyons 2019 for an overview.
 Gordon n.d.
 See e.g. Frances and Matheson 2019: § 5.
 Fricker 2007; Alcoff 2010.
 See e.g. Stanley 2005.
 Prichard et al. 2019.
 See e.g. Cohen 1986; DeRose 1992.
 Butchvarov 1970.
 Quine 1969; Kornblith 2002.
 For excellent overviews, see Steup 2019; Truncellito 2019; Steup, Turri, and Sosa 2013; BonJour 2002; and Huemer 2002.
 Perhaps in order to know whether one has acquired any knowledge, one must know what knowledge is and how we achieve it. This is the basis of Descartes’s 1984  strategy.
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The Gettier Problem & the Definition of Knowledge by Andrew Chapman
External World Skepticism by Andrew Chapman
Descartes’ “I think, therefore I am” by Charles Miceli
Moral Testimony by Annaleigh Curtis
Take My Word for It: On Testimony by Spencer Case
Conspiracy Theories by Jared Millson
Modal Epistemology: Knowledge of Possibility and Necessity by Bob Fischer
The Problem of Induction by Kenneth Blake Vernon
The Epistemology of Disagreement by Jonathan Matheson
Expertise by Jamie Carlin
Epistemic Injustice by Huzeyfe Demirtas
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About the Author
Tom Metcalf is an associate professor at Spring Hill College in Mobile, AL. He received his PhD in philosophy from the University of Colorado, Boulder. He specializes in ethics, metaethics, epistemology, and the philosophy of religion. Tom has two cats whose names are Hesperus and Phosphorus. shc.academia.edu/ThomasMetcalf