The Classical Definition: Justified True Belief (JTB)
Plato's dialogues (the Meno and the Theaetetus) first floated the idea that knowledge is true belief plus an "account" of why it's true, though the Theaetetus ends without accepting it. By the mid-20th century, philosophers commonly analyzed knowledge using a formula known as Justified True Belief (JTB).
According to the classical JTB model, you possess knowledge of a proposition (let's say, "It is raining outside") if and only if three strict conditions are met:
- Truth: The statement must actually correspond to reality (it must actually be raining). You cannot "know" something that is false; you can only mistakenly believe it.
- Belief: You must personally believe that the statement is true. You cannot "know" a fact if you secretly doubt or deny it.
- Justification: You must have adequate evidence, reason, or proof for your belief (such as standing outside and feeling raindrops).
To understand why justification matters, consider a person who buys a lottery ticket and claims: "I know this ticket is going to win." If the ticket happens to win, did the person "know" it beforehand? Philosophers answer no: it was a lucky guess, not knowledge, because the person lacked adequate justification at the time.
Descartes' Radical Doubt: "Cogito, Ergo Sum"
In his 1641 masterpiece Meditations on First Philosophy, the French philosopher René Descartes undertook one of the most famous intellectual experiments in history. Disturbed by how many false beliefs he had accepted during his youth, Descartes decided to demolish his entire belief system and start again from an unshakable foundation.
To do this, he adopted methodological skepticism: doubting every belief that possessed even the slightest possibility of error:
- Sensory Illusion: Our senses sometimes deceive us (a stick looks bent in water; distant towers look round when they are square). Therefore, sensory evidence alone cannot guarantee absolute certainty.
- The Dream Argument: How do you know with 100% certainty that you are awake right now and not experiencing an ultra-vivid dream?
- The Evil Demon (Malin Génie): Imagine an all-powerful, malicious demon whose sole purpose is to deceive your mind about everything—including geometry, mathematics, and the existence of your physical body.
After stripping away all uncertain beliefs, Descartes arrived at one single, indestructible truth that survived even the Evil Demon: In order to be deceived, doubted, or questioned, there must be an "I" that is doing the thinking.
Descartes expressed this foundational certainty in the phrase Cogito, ergo sum ("I think, therefore I am"), first written in French in his Discourse on the Method (1637) and in this Latin form in his Principles of Philosophy (1644). Even if everything else is an illusion, your own conscious awareness is undeniably real.
You can doubt whether the external world exists, but you cannot doubt that you are currently doubting.
Rationalism vs. Empiricism: Reason or Senses?
Descartes' work sparked the great epistemological debate of the Enlightenment between two opposing schools of thought:
1. Rationalism (Descartes, Spinoza, Leibniz)
Rationalists argued that the primary source of human knowledge is reason, logic, and innate intellect, independent of sensory experience (a priori knowledge). Sensory evidence is messy, subjective, and prone to illusion; true knowledge (like mathematics, geometry, and logical deduction) exists in the mind prior to sensory experience.
2. Empiricism (Locke, Hume, Berkeley)
Empiricists rejected innate ideas. John Locke proposed that the human mind is born as a Tabula Rasa (a blank slate). All knowledge, concepts, and ideas are written onto this blank slate through sensory experience and observation (a posteriori knowledge).
The Scottish empiricist David Hume took this logic to its radical conclusion: he pointed out that we never actually observe "causality" directly. When one billiard ball hits another, we see Ball A move, and then we see Ball B move. We do not see the invisible force of cause and effect; we merely infer it through habit. Hume demonstrated that even scientific induction relies on an unprovable assumption: that the future will resemble the past.
The Gettier Problem and Pragmatic Truth
In 1963, an American philosopher named Edmund Gettier published a brief three-page paper that shattered the Justified True Belief model. Gettier presented clever counterexamples showing that a person can have a Justified True Belief that is true purely by luck:
- Example (a stopped-clock case Bertrand Russell gave in 1948, now the standard way to illustrate Gettier's point; Gettier's own cases involved job candidates and coins): You walk into a room, look at a wall clock that reads 3:00 PM, and form the belief that it is 3:00 PM. Unknown to you, the clock broke exactly 24 hours ago. However, by pure coincidence, the actual time happens to be exactly 3:00 PM when you look. Your belief was true, and you were justified in trusting the clock—yet you did not genuinely "know" the time; you got lucky.
Gettier's paper launched a modern race to find a "fourth condition" for knowledge, such as reliability, undefeated justification, or virtue epistemology.
Meanwhile, American Pragmatists (William James, Charles Sanders Peirce, John Dewey) offered an alternative approach: instead of hunting for abstract, unassailable certainty, pragmatism defines truth by its practical consequences and working value. A belief is "true" if holding it allows us to navigate reality, solve problems, and predict outcomes successfully.
Takeaways
Epistemology investigates the foundations, justification, and limits of human knowledge. René Descartes demonstrated through methodological doubt that even if sensory experiences are illusions, conscious awareness itself is undeniable ("Cogito, ergo sum"). Knowledge requires balancing rational logical deduction (Rationalism) with empirical sensory observation (Empiricism), while recognizing that justification must be reliable enough to exclude lucky coincidences.
Action step: Choose a claim you read today in the news or social media, and evaluate it through the JTB lens: Is it demonstrably true? Do you believe it? And what specific, reliable evidence justifies that belief?
Sources & Further Reading
Sources
- René Descartes' Meditations on First Philosophy (1641). David Hume's An Enquiry Concerning Human Understanding (1748). Edmund Gettier's paper "Is Justified True Belief Knowledge?" (Analysis, 1963). Bertrand Russell's stopped clock: Human Knowledge: Its Scope and Limits (1948). Plato's Theaetetus 201c–210b. William James's Pragmatism (1907).
Go Deeper
- 📘 Meditations on First Philosophy — René Descartes (the foundational text of modern Western epistemology)
- 📘 Epistemology: A Contemporary Introduction — Robert Audi (a clear, thorough textbook covering modern knowledge theory)
- 🌐 Stanford Encyclopedia of Philosophy — Epistemology
- 🌐 Related piece in this library: The Biases That Fool Everyone
For Your Vocabulary
Epistemology (the branch of philosophy that studies the nature, origin, and limits of knowledge) · Justified True Belief (the classical three-part definition of knowledge) · Methodological Doubt (Descartes' systematic process of doubting all uncertain beliefs) · Cogito, ergo sum ("I think, therefore I am") · Rationalism (the view that reason is the primary source of knowledge) · Empiricism (the view that sensory experience is the primary source of knowledge) · Tabula Rasa (the concept of the mind as a blank slate at birth) · Pragmatism (philosophy assessing the truth of beliefs by their practical success)