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Philosophy 2: Justified True Belief (JTB)1. Truth: P is True2. Belief: S Believes That P is True3. Justification: S is Justified in Believing That P is TrueThe Gettier Problem
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# Philosophy 2: Justified True Belief (JTB)§

In classical epistemology and Western philosophy, particularly stemming from Platonic dialogues and later formal analytical traditions, **Justified True Belief (JTB)** served as the standard definition of knowledge.

Epistemology is the branch of philosophy concerned with the nature, grounds and limits of knowledge. What is knowledge? Different traditions define it in distinct ways:

In classical analytical philosophy, however, the definition of knowledge states that an agent $S$ **knows** that proposition $P$ is true if and only if:

  1. 1. **Truth:** $P$ is true.
  2. 2. **Belief:** $S$ believes that $P$ is true.
  3. 3. **Justification:** $S$ is justified in believing that $P$ is true.

To understand why all three conditions are necessary, consider agent $S$, who claims to know that **"the current time is 3:00 PM"** ($P$).

## 1. Truth: $P$ is True§

For a proposition to count as knowledge, it must correspond to objective reality. One can believe a falsehood, but one cannot *know* a falsehood.

**Assume:**

**Example:**

> It is actually 4:15 PM ($P$ is false). However, $S$ glances at his wristwatch, which was inadvertently set to a different time zone earlier that day, showing 3:00 PM. Trusting his watch, $S$ reasonably forms the belief that it is 3:00 PM.

Even though $S$ genuinely holds the belief and has reasonable justification for it, $S$ cannot be said to *know* that it is 3:00 PM.

Because the proposition itself is false, it can never constitute knowledge.

## 2. Belief: $S$ Believes That $P$ is True§

Knowledge requires cognitive assent. Even if a statement is true and evidence exists, an agent cannot be said to know something they do not actually believe.

**Assume:**

**Example:**

> It is indeed 3:00 PM ($P$ is true). However, $S$ has been working intensely in a windowless room and feels as though the afternoon has barely started. When a colleague remarks that it might already be 3:00 PM, $S$ dismisses the remark, convinced it cannot be past 1:30 PM.

Even though the fact remains true in the real world, $S$ does not know it is 3:00 PM.

One cannot possess knowledge of a truth while actively doubting or disbelieving it.

## 3. Justification: $S$ is Justified in Believing That $P$ is True§

Knowledge requires adequate epistemic grounding or rational evidence. A belief that turns out to be true purely by coincidence or a lucky guess does not count as knowledge.

**Assume:**

**Example:**

> It is precisely 3:00 PM ($P$ is true). $S$ is trapped in an isolated room with no clock, phone, sunlight, or external cues. Out of nowhere, simply because 3 happens to be his lucky number, $S$ blurts out, "I believe it is 3:00 PM right now."

In this case, $P$ is true, and $S$ truly believes it. However, because $S$ arrived at this belief without valid evidence or a reliable epistemic process, he did not *know* it - he merely made a lucky guess.

## The Gettier Problem§

For centuries, Justified True Belief was thought to be both necessary and sufficient for knowledge. However, philosophers (notably Edmund Gettier in 1963) revealed that one can possess a justified true belief that still fails to be genuine knowledge due to epistemic luck.

Consider this classic scenario:

> Imagine agent $S$ walks into a hallway at 3:00 PM, looks up at a clock on the wall, and sees that it reads 3:00 PM.

**The Twist:** The clock stopped working exactly 12 hours ago (at 3:00 AM).

$S$ held a Justified True Belief, but did $S$ actually know it was 3:00 PM? Most people say no - $S$ just got lucky that he walked by at the exact minute the dead clock happened to be right.

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