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Philosophy Questions: What Is the Definition of Knowledge?

Photo of Daniel McCoyDaniel McCoy | Bio

Daniel McCoy

Daniel is happily married to Susanna, and they have 3 daughters and 2 sons. He is the editorial director for as well as an online adjunct instructor for Ozark Christian College. He has a bachelor’s in theology (Ozark Christian College), master of arts in apologetics (Veritas International University), and PhD in theology (North-West University, South Africa). His books include the Popular Handbook of World Religions (general editor), Real Life Theology: Fuel for Effective and Faithful Disciple Making (co-general editor), Mirage: 5 Things People Want From God That Don't Exist, and The Atheist's Fatal Flaw (co-authored with Norman Geisler).

What is the definition of knowledge? The historic definition is that knowledge is “justified true belief.” This means that we know something if 1) it’s true, 2) we believe it, and 3) we have good reason to believe it (i.e., we are “justified” in believing it). Since everybody believes that what they believe is true, the real question becomes, how do we know that our beliefs are justified?

Some skeptical people have doubted that we can really know much of anything. We’ll read about some of them later in the article. Most people, however, feel confident that we can have lots of justified true beliefs (even if they don’t articulate it that way).

But how do we know what makes our beliefs “justified”? The two main ways that people feel they can truly know things are through 1) their ability to reason/be rational, and 2) their physical senses (“empirical” data). This article will tell the story of the European philosophers after the Renaissance who explored what makes our beliefs justified. Some of them focused on our ability to reason (the school of “Rationalism”) and others focused on our physical senses (the school of “Empiricism”).

Before we get to this important chapter in the history of philosophy, let’s unpack exactly what we mean by knowledge.

Definition of Knowledge: Justified True Belief

What is the definition of knowledge? Since the time of Plato, knowledge has been defined as “justified true belief.” Of those three words, “true” and “belief” are pretty easy to understand. You know that 2+2=4 because it’s true and you believe it. But according to the traditional definition, knowledge is more than just true belief; it’s justified true belief.

So, what does “justified” mean? It means you have good reason to believe it. You can back up your belief with sufficient evidence.

If I know that the earth is round, this means that…

  • I believe it.
  • It’s true.
  • I can give good reason for why it’s true.

One of Plato’s dialogues (Theaetus) narrates a conversation between the philosopher Socrates and one of his disciples, a mathematician named Theaetus. In it, they are exploring what knowledge is. At one point, Theaetus proposes that knowledge should be defined as “true belief.” Socrates pushes back by pointing out that sometimes people have accidental true beliefs—and that such cases shouldn’t count as knowledge.

“Socrates pushes back by pointing out that sometimes people have accidental true beliefs.”

I’m reminded of a scene in Muppet’s Treasure Island (the best Muppet movie ever made, of course) in which Fozzy Bear inspects a treasure map to see if it’s genuine. “Well, gentlemen,” he says, “this is definitely a genuine bona-fide treasure map!” When they respond with excitement, he says, “Yes. Mr. Bimbo told me so. Oh, Mr. Bimbo lives in my finger.” As it turns out, it was a genuine treasure map (Fozzy had a true belief), but it wasn’t a justified true belief. It was a true belief he accidentally happened to have, not based on good reasons he had for believing it. So, it wasn’t really knowledge.

Here’s another humorous example of accidentally true belief from Does the Center Hold: An Introduction to Western Philosophy by Donald Palmer:

“Do you believe that the earth is round?”



“Have you seen the bottoms of your feet? The only reason God would have created them round like that is if the earth was round.”

Is it true that the earth is round? Yes. Did he believe it? Yep. But his reasons for believing it weren’t good. It didn’t fit the definition of knowledge: it wasn’t justified true belief.

“His reasons for believing it weren’t good. It wasn’t justified true belief.”

A Challenge to the Historic Definition of Knowledge

So, that’s a helpful definition of knowledge that’s worth memorizing: justified true belief (JTB). However, the definition hasn’t gone unchallenged. A 20th century philosopher named Edmund Gettier has shown that, in rare cases, it’s actually possible to have justified true belief—but not necessarily for it to count as knowledge.

Here’s one of Gettier’s examples: Smith and Jones are applying for a job. The CEO tells Smith that Jones will get the job. Smith also discovers that Jones has ten coins in his pocket. Smith puts together the evidence and arrives at the belief that a person with 10 coins in his pocket will get the job. Here’s the twist: Smith will actually end up getting the job, and Smith also happens to have ten coins in his pocket. So, Smith’s belief that the job will go to a person with 10 coins is true and justified (Smith has good reasons for believing this). And yet, does this really count as knowledge?

“Gettier opens up the possibility that, at least in some eccentric examples, knowledge might take something more nuanced than merely justified true belief.”

Gettier thus opens up the possibility that, at least in some eccentric examples, knowledge might take something more nuanced than merely justified true belief. Even in such scenarios, knowledge still takes justified true belief. As Richard Knopp explains in Truth About God: What Can We Know and How Can We Know It?[1], JTB are necessary elements of knowledge, even if there’s a strange case or two in which they’re not completely sufficient. So, justified true belief (JTB) remains a helpful definition.

But How Do We Know Our Beliefs Are Justified?

Again, true and belief are pretty easy to understand. Something is true if it corresponds to the way things really are. The statement “The moon orbits the earth” is true because the moon, in fact, orbits the earth. A belief is something a person takes to be true. The idea of “belief” is easy to understand because we each have thousands upon thousands of beliefs we hold with varying degrees of intensity or conviction.

The problem is that, even though we all believe our beliefs are true (that’s what it means to believe something), some of our beliefs end up being false. How do we know when a belief is true or false? This is where justification comes in. We need to be able to have good reason for our belief.

Definition of Knowledge: “How do we know when a belief is true or false? This is where justification comes in.”

That may be helpful, but it also raises another question: how do we know when our beliefs are justified enough? Fozzy Bear was convinced that his belief was true (which it was), but he was also convinced that he had good enough reason for believing it (which he didn’t). In his mind, he was justified in believing what he believed. There need to be some criteria for what really counts as justification.

This need for criteria is why we’re going to take the article a step deeper than our original question (“What is the definition of knowledge?”) and ask a very practical subquestion: Given the JTB definition of knowledge, how do we know our beliefs are justified?

During the Middle Ages, European philosophers, heavily influenced by Christianity, felt secure that they held justified true beliefs. Why? It’s because they believed in God. After all, if there’s a God who created us in his image and created the world around us, then we have good justification for believing that our beliefs about him and about our world can be true. For, if God exists and created us, then he has revealed all sorts of truth for us to know.

“If God exists and created us, then he has revealed all sorts of truth for us to know.”

That confidence began to unravel corresponding with the European Renaissance.

Skepticism: Can We Really Know Anything?

Europe was getting over a nasty divorce between theology and philosophy (we might call this a Middle-Ages crisis). It was time for a new start. Specifically, they called it a rebirth, or “Renaissance.” They discovered there were more fish in the sea than Christian scholastic theologians like Thomas Aquinas. During the Renaissance, Europe found itself enchanted when reacquainting with old flames like the Roman statesman Cicero and the Greek philosopher Plato. The Renaissance was a time of revived interest in ancient Greco-Roman art, literature, and philosophy.

Too many flames can get overwhelming, however. With so many philosophical voices calling out for attention, some felt that it was better to take a moderate, nonpartisan approach toward the many claims to truth. A 16th century thinker named Montaigne learned from the ancient skeptic Sextus Empiricus the value of taking a more agnostic approach to knowledge.[2] The skeptic taught Montaigne (and Montaigne taught his generation) the unperturbedness of suspending judgment on questions of what is true. It is more conducive to one’s peace of mind to stay out of the vortices of dogmatic debate swirling all around.[3]

“The skeptic taught Montaigne the unperturbedness of suspending judgment on questions of what is true.”

Such people opt for skepticism with an amused smile. After all, windmills will keep turning round and round regardless of the Don Quixote’s transfiguring them into giants to be slain. These debates will never come to rest, so we might as well just sit back and enjoy watching the windmills.

What is preserved by such logic is one’s mental peace. But what is sacrificed is that now, we cannot really know anything.

And knowing that you don’t know anything presents a couple problems. First, it means you do know something, because you know you don’t know anything. I am not accusing Sextus Empiricus of going quite this far, but skepticism—if taken to its extreme—defeats itself. At the very least, skeptics, to be consistent, ought to be skeptical about their skepticism.

Second, realizing that you don’t know anything can actually really bother you, instead of bringing you the peace of mind it promises, especially if you really care about knowing.

Definition of Knowledge: With so many competing options, can any of our beliefs be justified?

Descartes: We Can Know with Certainty!

Rene Descartes, a French Catholic, was one such bothered person. He had read Montaigne and realized that skepticism had the dark power to turn all knowledge into doubt.[4] In college, Descartes had discovered the invincibility of mathematical knowledge. Mathematical truth was constant throughout the ages, whereas philosophers such as Aristotle were constantly being revised and rejected. Thus, Descartes would write,

“We should busy ourselves with no object about which we cannot attain a certitude equal to that of the demonstrations of arithmetic and geometry.”[5]

But where do you start if you are looking for certainty? Our five senses can deceive us. Even facts about nonphysical things, like the objects of mathematics, could be false, because our minds could be toyed with this very moment by an evil demon so that everything we think is an illusion.[6]

“Where do you start if you are looking for certainty? Our five senses can deceive us.”

 So in seeking certainty, Descartes was forced to doubt everything. Descartes thought about all the doubting he was doing, and that’s where he found his solution. All this doubting has to come from somewhere—from someone. There was the certain truth he had been looking for: “I think…therefore I am!” It was unmistakable, totally certain, that he existed, because he was the one doing the doubting. As Descartes put it, “I assuredly existed, since I was persuaded…I exist, since I am deceived…this proposition I am, I exist, is necessarily true each time it is expressed by me, or conceived in my mind.”[7]

That I am does not, however, tell me what I am. So Descartes started to build upon the certainty he had discovered. He could still be deceived, he reasoned, in thinking that he is a man or in his conception of what a man is. A demon could still be deceiving him into sensing a body he did not actually have or in experiencing sensations from a world that is not there. But was there anything he could know about himself for certain? “I am—I exist: this is certain; but how often? As often as I think….I am therefore, precisely speaking, only a thinking thing, that is, a mind.”[8]

So his newfound certainty yielded its first insight: I am a mind. But he realized that, just because he is a mind, a thinking being, does not mean that his every thought is necessarily true. Again, a mischievous demon could be having all sorts of fun filling our minds with illusions. So, are there any thoughts he could have that would have to be true?

“His newfound certainty yielded its first insight: I am a mind.”

Well, thought Descartes, what about the idea of God? I am finite, everything around me is finite, and yet I have an idea of the infinite. I am imperfect, everything around me is imperfect, and yet I have an idea of the absolutely perfect. Where could my notion of God come from other than from God himself?[9] Yes, thought Descartes, my idea of God ends up proving the existence of God. After all, it is impossible to think of a perfect God not existing, because that would be to lack the perfection of existing![10] God must exist, concluded Descartes. It’s another certainty. (This was a version of the “ontological argument” for the existence of God, which Christian apologists have mixed feelings about, many questioning its logic.)

Thus, through his ontological argument for the existence of God, Descartes went from doubting everything, to believing in his own self, to believing in God. Once past that threshold, things began speeding up. After all, if God exists and is perfect, and if it is an imperfection to deceive, then God would not deceive us about the world around us.[11] We could know all these things, with a knowledge perched safely on certainty.

“Descartes was understandably very satisfied with single-handedly saving knowledge from skepticism.”

Descartes was understandably very satisfied with single-handedly saving knowledge from skepticism. Thanks to his system of certainty, now we could believe in the self, God, the external world, and basically everything in between.

There was a problem, however. Early in his quest for certainty, Descartes had established that we are thinking things. Our bodies came to be proved later with the rest of the material universe, after God was proved. But we are very distinct minds. As time went on, the inevitable puzzle emerged: How is it that a distinctly disembodied mind is able to interact with a body?

Definition of Knowledge: Do we know our beliefs are justified only when we have 100% certainty?

School of Rationalism: Knowledge Is Justified Through Reason

His successors worked on the mind-body interaction problem. According to Gottfried Leibniz, God had ordered body and mind from creation with “pre-established harmony.” According to Nicolas Malebranche, God ordained laws so that, at the occasion of a change in our bodies, a corresponding change takes place in our minds.[12]

Benedict Spinoza, a Spanish Jew, had his own solution to the mind-body problem. Starting with the same premises, Spinoza constructed an opposite system. He taught that substance, like Descartes’ God, exists necessarily: “Existence belongs to the nature of substance.”[13]

Descartes had trained his successors to think in terms of “clear and distinct”: the mind had to be clear and distinct from the body, because that was the way we knew the mind existed for certain (I know for certain that I am a thinking thing). Here was Spinoza’s logic from there: If it is God’s nature to exist, and all other individual things must remain clear and distinct from God, then all individual things other than God couldn’t exist, because only God, as clear and distinct, could have existence.

“Here was Spinoza’s logic from there: If it is God’s nature to exist, and all other individual things must remain clear and distinct from God, then all individual things other than God couldn’t exist.”

In other words, Spinoza was a pantheist, believing that the universe and God are one and the same. In his system, there was no longer the problem in showing how mind and body interact, because mind, body, God, and everything in between are all made of the same divine substance. Nature moves according to the divine necessity.[14] Suddenly, we realize that Descartes’ system, however clever it might have been, was not nearly as certain as he had proclaimed it to be. After all, Spinoza, building on the same system, had arrived at an opposite “certainty.”

And that was only the discord inside Descartes’ circle. Descartes and Spinoza were in the philosophical school called “Rationalism” because they believed knowledge starts with reason. That is, one discovers knowledge by first laying out a foundation of certain, undoubtable truths and logically deducing the rest of the system from there. Recall Descartes’ quest for a single certainty from which to construct his system.

Definition of Knowledge: Is knowledge justified when reason is its foundation?

School of Empiricism: Knowledge Is Justified Through the Senses

A new school was taking the stage, however. The 17th century Englishman John Locke had read Descartes and wasn’t convinced that there were any ideas implanted in the mind by God. Rather, Locke believed he could prove that every thought we think comes to us from the world around us, from our sense experience. Thus, instead of being a rationalist, Locke was from the school of “Empiricism,” (which comes from the Greek word meaning “experience”). Knowledge comes to us not from reason but from experience.[15]

We can almost see history repeating itself in the Rationalism-Empiricism debate. It was Plato who was convinced that we are pitiable creatures, chained to our senses like prisoners in a cave, until we can break free to the mouth of the cave and behold real, eternal truths.[16] Then his student Aristotle came after him and taught that eternal truths are actually not to be found in some higher world, but are to be found all around us in this world. We extract the eternal Forms (Plato’s word for unchanging truths) as we study the particular objects around us.[17] In other words, the seeds of the Rationalism-Empiricism debate were planted in ancient Greece.

“The seeds of the Rationalism-Empiricism debate were planted in ancient Greece.”

 As we head back to Locke from ancient Greece, let’s make one stop along the way. Thomas Aquinas’s stately synthesis between theology and philosophy came stumbling down as William of Ockham attacked its foundation, casting doubt on whether the arguments for God’s existence were totally certain.[18] It is just as well for theology, thought Ockham, that it be separated from philosophy. After all, God doesn’t need to create according to “eternal essences” or “Forms.” God can do whatever he wants, and we don’t need to be cluttering up the path of his omnipotence with philosophical baggage. Why complicate things? God creates what he wants and how he wants. One cause is all we need, and that cause is God. Why multiply causes unnecessarily?[19]

This commonsense notion of sticking with the simplest explanation instead of adding in unnecessary causes is thus called “Ockham’s Razor,” or the Principle of Parsimony. The empiricists were zealous wielders of Ockham’s Razor. John Locke reasoned that it was simpler to get rid of ideas imprinted in the mind (often called “innate ideas”), in favor of what we can figure out from physical particulars around us. Man is a blank slate whose knowledge comes from the external world.[20]

When the empiricist George Berkeley, an Irish Anglican bishop, was bequeathed Locke’s physical particulars, he figured he could take Ockham’s Razor a slash deeper. Whereas Locke had gotten rid of innate ideas in favor of physical particulars, Berkeley reasoned that it was simpler to get rid of physical particulars in favor of immaterial mind. In other words, it is simpler for there to be no external world at all and for everything to simply exist in the mind. Berkeley believed in God and figured that there is no need for an external world when God could simply create the ideas of the world in our minds.[21]

“Berkeley reasoned that it was simpler to get rid of physical particulars in favor of immaterial mind.”

As radical as Berkeley’s proposal was, the third British empiricist, David Hume, would wield Ockham’s Razor a slash deeper still. But before we get to Hume, let us travel back to Descartes for a moment. Descartes was bound by his doctrine of distinctness (recall the separation of mind into disembodied “thinking thing”) to take away from material things anything resembling a mind. Minds have intentionality; minds can make things happen. In preserving the mind’s distinctness, Descartes needed a completely mechanical material world.[22]

Motion was therefore a problem for Descartes’ system. According to Descartes, material things could not be movers; they could only be moved. Things that animate from within confuse the system. What Descartes needed was a completely mechanical world whose motion could be explained entirely by the initial jolt by which God had started the world. Thus, cause-effect relationships within the system were an illusion, except insofar as God had set everything in motion at creation.[23]

Now, back to Hume. Whereas Descartes believed God certainly exists, Hume was a skeptic about God. Descartes had done away with causal relationships within the system, but, for Descartes, the notion of God was always there to explain cause-effect relationships as a result of creation. With God’s existence in question, cause-effect relationships have no certainty. After all, there is no certain relationship between kicking the stone and hurting one’s foot. In turn, since there is no certainty about cause-effect relationships, we can never prove God’s existence, because we can no longer trace the effects back to their First Cause.

Hume reasoned that without causality and without God, all we have left in our experience is a succession of apparently unrelated fragments.[24]

Definition of Knowledge: Is knowledge justified when sense experience is its foundation?

And We’re Back to…Can We Really Know Anything?

It is in the writings of Hume that we find empiricism in its most natural form. For as a skeptic, Hume cared not one iota about protecting previous metaphysical or theological systems. With nothing but experience as the starting point, Hume arrived at nothing but experience at the finish line—no mind, soul, deity, substance, or even causality. It becomes apparent that Hume intended to play the role of destroyer over protector.

In short, as if Descartes’ system was not already in trouble, it was hacked to death by wielders of Ockham’s Razor. Locke got rid of innate ideas in favor of physical particulars. Berkeley got rid of physical particulars in favor of immaterial mind. Hume got rid of immaterial mind in favor of mindless matter.

And the system that began in Montaigne’s skepticism ended with Hume’s skepticism.

“The system that began in Montaigne’s skepticism ended with Hume’s skepticism.”

 Descartes had believed that the only way to answer skepticism was to provide 100% certainty. But that is not just a tall order: it is a completely unnecessary one. Imagine a miraculous “weeping statue” which weeps—as long as you aren’t watching it. The moment you look, it’ll stop.

Now, can you prove my weeping statue wrong with 100% certainty? Not really. Because no matter what you say, I can always come back with some crazy story that could be true. (I mean, it’s technically possible that there could be a statue like that.) And neither could Descartes disprove the possibility that a demon is toying with our brains so that we can’t actually trust our senses. But “Can you prove me wrong?” is the wrong question to be asking.

If I believe something (e.g., the weeping statue, the demon toying with the brain), then I have the burden of proof to give you reasons why it is true. You shouldn’t have to grant that something is true just because you can’t prove it wrong. If I want to get you to believe something, I need to give you positive reasons to believe. It is completely unfair for me to expect you to believe something just because you can’t disprove it with 100% certainty.

“You shouldn’t have to grant that something is true just because you can’t prove it wrong.”

So, are there any reasons why we should believe a demon is controlling our minds and that the world is an illusion? We cannot disprove such a notion with 100% certainty, but we shouldn’t have to. Sure, Hume was correct that we cannot prove cause-effect relationships with complete certainty, but is there any reason to doubt the connection between the overhead cloud and the rain, between the sun and the warmth?

The search for certainty will not lead to knowledge, but to skepticism. So, we shouldn’t go in search of certainty, but rather in search of where the evidence points.

Definition of Knowledge: When we set the bar for justification at certainty, what happens to the quest for knowledge?

Knowledge Is Possible (Even If Certainty Isn’t)

So, back to the question we’re wrestling with: Giving the JTB definition of knowledge, how do we know when our beliefs are justified?

Is knowledge to be found in reason? In sense experience? A very helpful answer would come after Hume, and that answer can be seen as good news for both sides. The truth is, we don’t have to choose between reason and experience: knowledge consists of both. After Hume would come a thinker who would bring about a compromise between the two sides. His name was Immanuel Kant.

But Kant’s is a story for another time. What needs to be noted here is that, though Kant was right to accommodate both reason and experience, even Kant’s grand synthesis would itself end in its own brand of skepticism. While Kant would make room for both reason and experience, he did not make room in his system for metaphysical knowledge about God (although he did try to sneak God in through moral reasoning). Hence, Kant’s disciples would find themselves trudging through more skepticism.

Definition of Knowledge? “Kant was right to accommodate both reason and experience.”

But effects are traceable back to their cause. And if we can reasonably trace morality back to a moral law Giver, design to a Designer, and creation to a Creator, then we can have justified true belief about God and his creation. Can we have 100% certainty in these beliefs? No, but we can have sufficient knowledge because God has given us more than sufficient evidence that he is there.

And anyone who demands 100% certainty before they’ll consider whether our belief in God is justified is forgetting that, as Descartes discovered early in his philosophical experimentation, the only thing a person can know for 100% certain is that they themselves exist. That’s it. Again, the quest for certainty aside, our belief in God is validated by both good reason and experience—our own experiences, as well as of those who directly experienced acts of God in biblical history.

If we, as humans created in God’s image, participate in a measure of God’s rationality, then knowledge is his gift to us, a way to enjoy his world and seek his truth. If God created us in his image, then we participate in rationality (i.e., the rationalists were onto something) as we discover his world (i.e., the empiricists were onto something).

“If God created us in his image, then we participate in rationality as we discover his world.”

The reality of God makes skepticism seem illogical and makes the pursuit of knowledge less the anxiety of a red-faced arguer and more the contentedness that comes from each successful connection of the jigsaw puzzle. It’s his world, and it’s our joy to discover how its pieces fit together.

[1] Richard A. Knopp, Truth About God: What Can We Know and How Can We Know It? (, 2021), footnote 45.

[2] John Cottingham, “Montaigne, Michel Eyquem de,” in The Oxford Companion to Philosophy, ed. Ted Honderich (Oxford: Oxford University Press, 2005), 618-619.

[3] Philip P. Hallie, “Sextus Empiricus,” in Encyclopedia of Philosophy, Vol. 7, ed. Paul Edwards (New York: Macmillan Publishing, 1972), 427-428.

[4] Etienne Gilson, The Unity of Philosophical Experience (San Francisco: Ignatius Press, 1999), 101-102.

[5] Gilson, 103-105.

[6] Rene Descartes, “Meditation I,” Meditations, trans. John Veitch

[7] Descartes, “Meditation II.”

[8] Descartes, Meditation II.

[9] Descartes, Meditation III.

[10] Descartes, Meditation V.

[11] Descartes, Meditation IV.

[12] Gilson, 148-149.

[13] Baruch Spinoza, “Part I: Concerning God,” in Ethics Demonstrated in Geometric Order, trans. R.H.M. Elwes

[14] Spinoza.

[15] Gilson, 134-135.

[16] Plato, “Book 7,” in The Republic, trans. Benjamin Jowett

[17] Ronald H. Nash, Life’s Ultimate Questions: An Introduction to Philosophy (Grand Rapids: Zondervan, 1999), 99.

[18] Frederick Copleston, Ockham to Suarez, in A History of Philosophy, Book One (New York: An Image Book, 1985), 12.

[19] Gilson, 58-61.

[20] Donald Palmer, Does the Center Hold? An Introduction to Western Philosophy, 3rd ed. (Boston: McGraw Hill, 2002), 82-83.

[21] Lisa Downing, “George Berkeley,” in The Stanford Encyclopedia of Philosophy (Spring 2013 Edition), ed. Edward N. Zalta

[22] Gilson, 160-161.

[23] Gilson, 168-169.

[24] Gilson, 173.