How Do We Know What We Know? An Introduction to Epistemology

"The only true wisdom is in knowing you know nothing." ~ Socrates 

Every decision we take, from trusting science to believing what we are told about history and everyday life, is a form of knowledge. But how can one know that this knowledge is true, or better yet, justified? How do we know what we know? This is what epistemology – the branch of philosophy concerned with the study of knowledge – seeks to answer, with philosophers spending decades upon decades trying to reach an agreed-upon conclusion. Understanding epistemology helps us distinguish between opinion and fact, certainty and doubt, and reliable evidence versus mere belief. This essay aims to introduce the different branches of epistemology while also exploring the different problems that epistemology faces, from the Gettier problem to skepticism, while also realizing why epistemology especially matters in the age of artificial intelligence and misinformation. 

"All men by nature desire to know." ~ Aristotle, Metaphysics

The word epistemology is derived from the Greek words epistēmē – knowledge – and logos – reason or study – and is commonly defined as the study or theory of the nature and grounds of knowledge, especially with reference to its limits and validity, according to Merriam-Webster. Epistemology aims to answer questions such as: What is knowledge? How do we acquire it? Can we be certain of what we know?

Epistemology extends beyond simply collecting or acquiring facts; it investigates the standards by which beliefs are formed, justified, and accepted as knowledge. For example, when a teacher is explaining a lesson and is relaying information to you, you naturally assume that this information has passed the process of becoming knowledge; however, when you read a new scientific article claiming to have discovered something that changes your perception of reality, you naturally ask for the evidence that supports the claim and whether the evidence is sufficient. This is what epistemology provides a framework for which we, as a society, evaluate, judge, and dissect knowledge. Therefore, it serves as a foundation across all disciplines, such as those of politics, science, and ethics. 

For decades, philosophers have asked why knowledge matters or is considered more valuable than simple true belief. Anyone can come about information or a correct conclusion by mere luck and coincidence, but philosophers argue that true knowledge takes more than luck. As such, knowledge is regarded as valuable because it connects to deeper issues about the mind, truth, and our relation to the world, while others also say that knowledge is inherently valuable. 

Knowledge is traditionally understood in Western philosophy as justified true belief (JTB), a view attributed to Plato. According to this standard, a person knows something only if they have a true proposition and a justified belief, in which case one can claim that they have acquired knowledge. Although this view is what shaped philosophy and the understanding of knowledge for centuries, it was later challenged by Edmund Gettier in 1963, after he introduced what is called the Gettier problem and claimed that a person may genuinely possess a justified true belief but still not have reached true knowledge. This changed contemporary philosophers' understanding of knowledge. 

"Knowledge is of things we see; belief is of things we do not see." ~ Plato (paraphrased from Republic)

Before examining how knowledge is acquired, one must first answer a more fundamental question: what qualifies as knowledge? Although the term knowledge is used extremely frequently in everyday life, defining it philosophically has proven far more difficult. Most individuals often claim to “know” something simply because they believe it to be true; however, belief does not necessarily constitute knowledge. One can believe a statement to be true even when one has arrived at that conclusion by pure luck or chance; conversely, a person may sincerely and confidently believe a statement even when it's false, but their confident belief does not make it true. As such, epistemologists distinguish between knowledge and belief by asking what conditions belief must satisfy to qualify as knowledge.

To understand the distinction, it is important to differentiate between truth, belief, and knowledge. A belief is any proposition a person may hold that they accept as true. This belief is either in fact true, meaning it reflects reality, or false, meaning it does not reflect reality. Beliefs are based on a multitude of things, from personal experiences, intuition, and assumptions to culture, environment, and testimony, while truth, on the other hand, is a proposition rooted in reality, meaning it must be logical and reasonable. Finally, any belief must be justified, meaning there must be adequate evidence, reasoning, and explanation for it to advance as knowledge. Justification is what one uses to differentiate between knowledge and mere opinion.

Plato's Allegory of the Cave, presented in the Republic, highlights the distinction between truth, belief, and knowledge. In the allegory, three prisoners are trapped in a cave (it is presumed that they have been there since birth or for as long as they can recall), and they are only able to see the shadows that are cast on the wall. Having no experience of the outside world or understanding of what these shadows are, they mistake them for reality. One day, one of the prisoners escapes and leaves the cave. He is met with a three-dimensional reality, one that isn't just shadows, and he then realizes what he previously assumed was true of reality was merely an illusion. Upon returning to the cave, he describes what awaits outside and gives the other prisoners his testimony; however, they reject his account because it contradicts everything they have ever known. Through this allegory, Plato argues a plethora of things, one of them being that genuine knowledge demands moving beyond appearances and opinion.

The theory of justified true belief dominated the epistemological field for centuries because it seemed to encapsulate all the requirements of knowledge. It provided a clear distinction between knowledge and opinion while emphasizing the importance of evidence, reason, and truth. Nevertheless, as philosophy developed, scholars began to question whether these three conditions truly satisfied the definition of knowledge. Could a person possess a justified true belief and still not have achieved genuine knowledge? This question was presumably answered until the twentieth century, when Edmund Gettier presented a series of thought experiments that proved that a person may satisfy all three conditions Plato proposed but still fall short in possessing genuine knowledge.

"Is justified true belief knowledge?" ~ Edmund Gettier (1963)

The Gettier problem is a challenge to the traditional idea of knowledge being a justified true belief. Previously, knowledge was viewed as such that a person knows P only if P is true, the person believes P, and then P is justified; however, in 1963, Edmund Gettier challenged this belief in his three-page paper titled Is Justified True Belief Knowledge?, which transformed the field of epistemology and philosophy as a whole.

Gettier argued that there are situations in which a person may possess a justified true belief but still lacks genuine knowledge. In the cases proposed by Gettier, the individual's belief is true and is supported by sufficient evidence, yet the truth of the belief is only true because of luck or coincidence rather than an accurate connection between the evidence and reality. These cases proved that the traditional definition fails to distinguish between genuine knowledge and accidental true belief. 

One of Gettier's most famous examples uses two job applicants, Smith and Jones. Smith has strong evidence that Jones will get the position, as the company president told him, and Smith has also noticed that Jones has ten coins in his pocket. As such, Smith infers that the applicant with ten coins in their pocket will get the job. Unknown to Smith, however, it is he who will get the job, not Jones; coincidentally, Smith also has ten coins in his pocket, making his claim a true, justified belief even if it was not what he intended or meant. This case, among many others, forced philosophers to reevaluate what knowledge was, as most philosophers agreed that knowledge could not be coincidental.

Gettier's thought experiments had profound and lasting implications for the field of epistemology, prompting philosophers to reevaluate their definition of knowledge. The thought experiments demonstrated how, even when the three conditions of Plato's definition of knowledge are met, one can still not have acquired knowledge, forcing philosophers to find other ways to evaluate knowledge, such as some saying knowledge requires a fourth condition, like the absence of epistemic luck, while others proposed entirely different theories to evaluate knowledge, such as including reliabilism, virtue epistemology, and knowledge-first epistemology. Although none of these theories has come to universal acceptance, they all aim to understand what knowledge is, and they have all vastly expanded the field of epistemology. 

The Gettier problem remains one of the most influential and progressive developments in modern philosophy, as it highlights the complexity and vastness of knowledge. It shows that possessing a justified true belief does not equate to possessing knowledge, as truth can sometimes come about by coincidence and/or luck rather than logical and reliable reasoning. As a result, Gettier's work shifted epistemology from asking whether the beliefs we hold are true and justified to how belief, truth, justification, and luck all shape what we count as knowledge.

Introduction to Sources of Knowledge 

"It is not enough to have a good mind; the main thing is to use it well." ~ René Descartes

Having established what knowledge is, one must know what precedes it or how one can acquire knowledge. Throughout history, epistemologists have debated the origins of knowledge and whether certain sources and/or theories are more reliable than others. Although no unified agreement has been achieved, most philosophers recognize, associate, and agree on one of several primary sources, such as perception, reason, experience, memory, testimony, and introspection. Each of these contributes to human understanding in some unique way while also posing different limitations that challenge the source's reliability. 

"Seeing is believing." ~ Traditional proverb

One of the most immediate and fundamental sources of knowledge is perception, the process through which individuals acquire information or knowledge about the external world using their five senses. Much of what people know comes from their five senses, from recognizing people's faces to realising its raining heavily relies on sensory perception. However, philosophers have long discussed whether the senses are a reliable source, as people are capable of hallucinating, experiencing optical illusions, and making general perceptual errors, leading to false beliefs based on sensory experience.

Plato, for one, believed that the senses are unreliable and unfit as a source of knowledge and that true knowledge came from reason rather than perception. In contrast aristotle believed that the senses could serve as a starting point for the physical world of knowledge, but not the metaphysical. In opposition, John Locke heavily defended perception as a source of knowledge, as he believed people are born as blank slates and that knowledge starts with perception, making perception central to his idea of human understanding. Similarly, in India, the Cārvāka (materialist) school believed that perception alone is a sufficient source of knowledge and that inference is uncertain at best.  

Although philosophers continue to disagree about the reliability of perception, few deny that it has a fundamental role in the world of knowledge. Whether it is regarded as a primary source of understanding or merely a starting point for further reasoning, perception provides the initial contact between the mind and the physical world. At the same time, however, the susceptibility perception has to illusion, hallucination, and error, as well as subjective interpretation, demonstrates how it alone cannot always guarantee truth. Consequently, many epistemologists argue that perception is most reliable when supported by other sources of knowledge such as reason, experience, memory, testimony, and/or introspection, allowing beliefs to be evaluated rather than being accepted for face value. 

"I am thinking; therefore I exist." (Je pense, donc je suis. Cogito, ergo sum.) ~ René Descartes

Reason refers to the ability to acquire knowledge through rational thought, logic, and deduction. Rather than depending on perception, reason allows one to come to a conclusion by drawing  and/or analyzing logical inferences. Mathematical truths, for example, are regarded as products of reason rather than perception since they are able to be proven through pure reason rather than needing empirical observation.

  One of the most regarded and well-known rationalists and skeptics is René Descartes, often referred to as the father of modern philosophy, as he tried to rebuild knowledge from a foundation that couldn't be doubted. He believed that reason was the most reliable path to knowledge and argued that certain knowledge could be attained through pure reason alone. Moreover, Baruch Spinoza was a major philosopher during the 17th century and believed that rationalism was the path to knowledge; however, he differed from Descartes in that he took reason in a more geometric and systematic direction. He believed that understanding things through adequate and sufficient ideas and logical necessity leads to freedom, because knowledge is what frees one from confusion and helps one overcome the power of the passions, at least for Spinoza. 

Despite its strengths, reason is not without its limitations. Logical reasoning is heavily influenced by the assumptions from which they begin, which can be deeply biased and influenced by a plethora of things. Therefore, reason alone cannot establish many facts about the physical world without support from observation and experience. "No man's knowledge here can go beyond his experience." ~ John Locke

While perception provides the immediate sensory information through which individuals observe the world, experience refers to the knowledge and understanding gained through repeated action with one's environment over time. Rather than relying on a single observation, experiential knowledge is formed by accumulating and reflecting on various and multiple experiences, allowing one to identify patterns, establish relationships, and make informed judgments. Additionally, experience transforms isolated perceptions into meaningful understanding, enabling one to apply past learning to new situations. 

Experience plays a fundamental role in our everyday life and scientific inquiry. From an early age, individuals learn about the world around them through lived experience. For instance, a child who touches a stove and realizes it's hot will learn not to touch the stove. It's only through repeated experience that this knowledge will be applied to other situations, and the child will learn to avoid other hot surfaces. Similarly, physicians refine their diagnostic abilities after treating numerous patients, and scientists discover new information and develop theories by repeatedly observing natural phenomena or by repeating experiments.

The significance of experience is most clear in the field of epistemology under empiricism, a belief that knowledge and information arise mainly from sensory experience. Philosophers such as John Locke rejected the idea that humans are born with innate ideas and instead believed that individuals are born as blank slates and that knowledge is gradually built through experiences. David Hume also believed this view and asserted that all ideas ultimately come from prior impressions (sensory or internal experiences); as such, for Hume, the only legitimate source of information for our ideas is experience, even if it cannot guarantee certainty. Their work, among others, had lasting implications on the development of the scientific theory that relies on observation, experimentation, and evidence rather than intuition or authority. 

Despite its importance, empiricism is not without its limitations. Individual experiences are often shaped by personal expectations, emotions, or cultural influences, making experiences extremely subjective, as such people interpret the same experience in vastly different ways, leading to differing conclusions. Furthermore, conclusions drawn from limited experiences can often lead to misinformation or faulty generalizations. This has led various philosophers to argue that experience should not be accepted without critique and instead be evaluated alongside reason, evidence, and other sources of knowledge. 

"Memory is the treasury and guardian of all things." ~ Marcus Tullius Cicero

Another notable faculty is memory; although some argue it is not a primary source of knowledge, and instead mainly allows individuals to retain and recall knowledge, others maintain that it can contribute to or generate knowledge. In either scenario, it has a key role in the field of epistemology. Much of daily life is heavily reliant on trusting memory, whether it's recalling a friend's birthday, recalling a historical event, or recognizing a familiar place. Philosophers have long questioned whether memory is merely a tool for preserving knowledge or if it's capable of producing new epistemic justification for knowledge, and the central question is expressed as follows: if one remembers that Athens is the capital of Greece, does one know that because one remembers it now, or because one previously acquired the knowledge and memory merely preserved it? 

At this point, an important distinction must be made between semantic memory and episodic memory. Semantic memory preserves knowledge about the world; however, when one recalls the knowledge, it is detached from how one acquired it, meaning one cannot remember whether they acquired it through testimony, experience, perception, or reasoning. It is merely the recollection of knowledge. On the other hand, episodic memory preserves not only knowledge of the event but the lived experience of the event. For example, one recalls that the pyramids are in Cairo, one would then say, “I remember seeing the pyramids in Cairo at sunset.” This differs from semantic memory in that the event is being recalled, providing knowledge of one's lived experience, while semantic memory just preserves knowledge of the world.

Nevertheless, psychological research has shown that memory is not always accurate. Memories can fade, and in fact, research shows that every time one recalls a memory, it is slightly altered. While memory is indispensable, its reliability often depends on the accuracy of the original event and the circumstances under which it is recalled. 

"We are all born ignorant, but one must work hard to remain stupid." ~ Benjamin Franklin

A considerable proportion of human knowledge is also acquired through what's known as testimony, which is a formal statement or a spoken proof that something is true. Only a handful of people ever verify every scientific, historical, or political event they believe. Instead, the information relayed to them by other people is assumed to be true. Modern society would virtually collapse if one stopped taking testimony as a source of knowledge, since no one can independently verify all the claims made and all the pieces of knowledge they possess. 

Again, philosophers maintain differing views on the reliability of testimony as a source of knowledge, with philosophers such as Aristotle treating credible reports and reputable opinions (endoxa) as important starting points in inquiry, though he still preferred firsthand rational insight or direct acquaintance over mere report. John Locke, on the other hand, heavily rejected the acceptance of testimony without independent support and believed that beliefs should rest on one's own evidence. Conversely, Thomas Reid pushes back against Humean skepticism, arguing that we have an innate trust in testimony (and perception) unless there have been reasons for doubt. 

Hindu epistemology (Nyāya) treats testimony (śabda) as an independent source of knowledge when the speaker is trustworthy and has proven reliable; as such, they don't believe testimony requires prior inferential confirmation. This differs from Buddhist epistemology (e.g., Dignāga, Dharmakīrti traditions) in that one must first reduce testimonial knowledge to inference, meaning one must infer the speaker's reliability before gaining knowledge from their words.

In all cases, testimony, while profoundly valuable, poses significant challenges from implicit bias, purposeful deception, and unreliable authorities that can all lead to false beliefs. For this reason, epistemologists emphasize the need to evaluate the credibility of sources and evidence before accepting testimony as knowledge.

 “Yesterday I was clever, so I wanted to change the world. Today I am wise, so I am changing myself.” ~ Jalal al-Din Rumi 

The final major source of knowledge is introspection, the process through which one examines one's own mental state, emotions, and thoughts. Although introspection does not necessarily provide knowledge about the world, it provides invaluable knowledge about oneself. Individuals generally possess direct awareness of experiences such as pain, happiness, fear, or doubt without relying on external observation. Philosophers have argued that introspection provides a unique form of self-knowledge that can't be attained otherwise. 

Descartes, for one, viewed introspection as a paradigm of certainty; the famous cogito “I am thinking therefore I am”  relies on this certainty. In addition, Locke and many early empiricists treat introspection as a basic source of ideas. Moreover, many contemporary philosophers refer to introspection as a highly reliable source for current conscious states (e.g., “I am mad,” “I am in pain”), but not infallible or immune to correction by external evidence. An important distinction to make is between causal process and phenomenal content, how it's like now,  where introspection is relatively reliable and brings value, whereas causal process (why I feel like this, what biases shape my judgments) is notably weak. 

Nonetheless, introspection is not infallible, as people may misunderstand their own motives, memories, or emotional responses. Modern psychology has demonstrated that unconscious biases and cognitive processes often influence human thought without direct conscious awareness, suggesting that introspection (self-knowledge) can also be limited.

Conclusion of sources of knowledge 

“Our knowledge can only be finite, while our ignorance must necessarily be infinite." ~ Karl Popper

Although each source contributes to human understanding and knowledge in a valuable way, none can stand alone. Perception may provide information about the external world, but it often requires reason and logic to accurately interpret the information. Memory preserves past experiences, yet it must be supported by reliable perception and reasoning. Likewise, enables the flow of information and knowledge, but must be accompanied by a reliable and trustworthy messenger. Consequently, epistemologists generally recognize that knowledge is acquired through a combination of all these sources rather than any one method. This recognition has given rise to enduring philosophical debates about the sources of knowledge and which should be considered the most fundamental and sufficient, leading to the contrasting traditions like that of rationalism and empiricism.

 "The fundamental cause of trouble is that in the modern world the stupid are cocksure while the intelligent are full of doubt." ~ Bertrand Russell

Since the emergence of epistemology as a distinct branch of philosophy in the 19th-century by the Scottish philosopher James Frederick Ferrier, who is widely accredited with distinguishing epistemology from metaphysics, logic, and psychology, scholars have not only debated what knowledge is but how it is acquired and justified. These disagreements and differences of opinion have given rise to different epistemological theories, each offering a unique and different understanding, approach, and evaluation of human knowledge. While no single theory has proven sufficient to stand alone and thus gain universal acceptance, the debates that have surrounded the theories have shaped the contemporary field of epistemology in lasting and profound ways.

One of the oldest and most influential debates is that between rationalism and empiricism. Rationalists argue that reason is the primary source of knowledge and that certain truths can be attained independent of sensory experience. In the ancient world plato is often read as a precursor to rationalism, privileging reason and logic as well as the intelligible realm of Forms over the senses, while Aristotle is considered the precursor of empiricism, maintaining that knowledge begins with the senses, experience, and observation.

In the 17th and 18th centuries, this debate widened and split into two broad categories: The Continental Rationalists and the British Empiricists. Rationalists like those of René Descartes, Baruch Spinoza, and Gottfried Wilhelm Leibniz emphasized that knowledge is primarily sourced and grounded in innate ideas and the intellect, which can directly grasp or derive logical and mathematical truths through deduction. Descartes, for one, believed that reason alone could prove sufficient to prove mathematical truths and the existence of the self, as, at least in his worldview, they are innate or discoverable through logical deduction. His famous declaration (better translated as “ I am thinking; therefore I exist” demonstrates how one can reach truth and knowledge through pure reason, even when all the sensory experiences are doubted (Descartes famously stated that "I will suppose... that everything I see is false." (from Meditations on First Philosophy)). Spinzo extended this confidence in that he constructed a fully deductive, geometric system in his Ethics, while Leibniz defended innate ideas and necessary truths known a priori, distinguishing them from contingent truths of fact. 

In opposition, empiricists like those of John Locke, George Berkeley, and David Hume argued that knowledge is not primarily innate and is best attained through careful observation of the physical world through sensory experience. Locke attacked and completely rejected the idea of individuals being born with innate ideas; instead, he describes individuals as being born with a blank slate (tabula rasa) shaped by sensation, experience, and environment. Berkeley pushed empiricism further by denying material substance and claiming that objects for so long as they are perceived (he, for example, would believe the tree doesn't make a sound when it falls if no one is around to perceive/hear it make the sound), while Hume pushed empiricism to its skeptical limits, questioning the rational basis of causation, induction, and the substantial self. This empiricist tradition grew stronger during the 17th-century, with figures such as Francis Bacon upholding induction and careful observation as the proper method for acquiring knowledge. 

The tension between rationalism and empiricism dominated the philosophical world through the Enlightenment and was far from resolved till the appearance of Immanuel Kant. Kant sought to connect both rationalism and empiricism by positing that while all knowledge begins with experience, it is structured by a priori concepts and forms of intuitions (such as space, time, and the categories), so that both reason and experience work together and are necessary for knowledge. Later developments, including logical positivism and other various forms of neutralized and pragmatist epistemology, continued to reframe the debate, but the core tension – between reason as a necessary source for the attainment of knowledge,  and universal truths, and experience as the foundation of contingent, empirical knowledge – remains central to the history of philosophy as well as the contemporary world of philosophy.  

"The important thing is not to stop questioning." ~ Albert Einstein

Although many epistemological questions remain largely unanswered, the discipline continues to shape philosophy and influence all fields, from politics to science, psychology, and artificial intelligence. In a world saturated with information, misinformation, and competing truth claims, the ability to distinguish between knowledge and mere belief or opinion, and to reach one's own conclusions, is more important than ever, and epistemology provides the intellectual tools to question, discern, and evaluate the beliefs we hold and the information that we encounter. Ultimately, the study of epistemology reminds us that the pursuit of knowledge is never-ending and not only about finding definitive answers, but also about continually questioning and understanding ourselves and the world around us. 

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