Understand Plato’s theory of knowledge in a simple way. These notes explain how Plato refutes relativism and skepticism, why knowledge is not sense-perception or opinion, and how knowledge becomes “Justified True Belief.” A clear guide for philosophy students.

Table of Contents

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Introduction to Plato and His Epistemology

• In the last lecture we studied Socrates; now we begin Plato, who will require multiple lectures.
• This section focuses on Plato’s epistemology, meaning his theory of knowledge — what knowledge is and how we can know something.
• Socrates died in 399 BC, and at that time Plato was about 30 years old.
• Socrates’ death deeply affected Plato, leading him to dedicate his entire life to philosophy.
• Plato considered Socrates the wisest and most just person, which he expresses in his dialogue Phaedo.
• Plato and his friends tried to stop Socrates’ death sentence but failed, leaving Plato disturbed by the injustice of society.
• Plato was born in 428/427 BC in Athens in a wealthy and aristocratic family, originally trained to be a political leader.
• After Socrates’ death, his direction changed completely toward philosophy instead of politics.
• Plato’s influence is foundational in Western philosophy; almost every later philosophical question can be traced back to him.
• Philosopher A.N. Whitehead stated that European philosophy is essentially a series of footnotes to Plato, meaning his ideas shaped the entire tradition.
• Plato’s most famous student was Aristotle, and most Western philosophers align broadly with either Plato or Aristotle.
• Around 387 BC, Plato founded his school named Academy, located outside Athens in a garden area linked to the hero Academus.
• Plato spent the rest of his life teaching and writing over 30 works, all of which are still available today.
• His most important work is The Republic, which is essential for understanding his philosophy and will be discussed later.

Summary

This section introduces Plato’s life background and how Socrates’ death led him to devote himself entirely to philosophy. It also explains his major influence on Western thought and mentions his school, the Academy, and his key work, The Republic.

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Philosophical Background Influencing Plato

• In Plato’s time, several philosophical ideas were popular: Parmenides’ idea of One (unchanging reality) and Heraclitus’ idea of constant change (flux) — both were opposite views.
• Democritus’ atomism left no space for morality or religion; everything was explained as material atoms.
• Sophists like Protagoras argued relativism (“man is the measure of all things”), and Gorgias promoted skepticism (“nothing is true”).
• There was also debate between physis (nature) and nomos (convention) — whether values are natural realities or just social rules.
• Amid this confusion, Socrates sought real truth, but society punished him with death.
• Plato wanted to continue Socrates’ ethical search for truth, especially about how to live a good and just life.
• Socrates’ death showed Plato that a good person cannot survive in a corrupt society; therefore, society must change first.
• So, Plato began thinking about the ideal state, where a good person can live a good life.
• Before explaining a good state, Plato needed to show that goodness is real and objective, not just a social convention.
• Sophists believed goodness is only nomos (a human-made rule), but Plato knew society’s opinion can be wrong, as seen in Socrates’ trial.
• Proving that goodness truly exists becomes the starting point of Plato’s philosophy.

Summary

This section explains the philosophical background that shaped Plato. He reacted against relativism and skepticism, aiming to prove that goodness is real. Influenced by Socrates’ death, Plato focused on creating a just society where good people can truly live.

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Plato’s Refutation of Relativism

• Plato first needed to prove that knowledge is possible, so he had to refute Relativism and Skepticism, which denied objective truth.
• Relativism (from Protagoras) claims: “Man is the measure of all things” — meaning whatever appears true to each person is true for them.
• According to relativism, truth depends on personal opinion; there is no objective truth that is the same for everyone.
• This idea is also applied to moral values, suggesting no fixed right or wrong, only personal or social opinion.
• In his dialogue Theaetetus, Plato gives arguments against relativism. One main argument is: Relativism refutes itself.
• If “every opinion is true,” then the opinion of someone who says “not every opinion is true” must also be true.
• This means Protagoras’ own claim proves itself false, because it allows its direct opposite to be equally valid.

Summary

Plato begins by proving that knowledge can exist. He challenges Protagoras’ relativism by showing that if every belief is true, then the belief that relativism is false must also be true. Therefore, relativism destroys itself and cannot stand as a valid theory.

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Plato’s Three Arguments Against Relativism

• Plato says Relativism contradicts itself. If Protagoras claims every opinion is true, then the opinion that relativism is false must also be true. So his own theory destroys itself.
• Sophists charged high fees for teaching. But if every opinion is already true, then no one needs teaching. This shows that even Sophists believed their opinions were better, which goes against relativism.
• Plato’s second argument: Two opposite claims cannot both be true at the same time.
  Example: If a person believes his leg is not broken, but a doctor says it is broken — one of them must be wrong. Truth cannot vary just by personal belief.
• The third argument: All opinions are not equal. Some opinions are based on knowledge and expertise.
  Example: A musician judges music better than a boxer; a doctor understands medicine better than a politician. So expert knowledge has more value than random opinion.
• Therefore, truth cannot be merely personal, and knowledge must have an objective basis.

Summary

Plato refutes relativism by showing that it contradicts itself, cannot accept opposite truths at once, and ignores the difference between knowledgeable and ignorant opinions. This helps Plato take the first step toward proving that real, objective knowledge is possible.

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Plato’s Refutation of Skepticism

• After refuting relativism, Plato also needed to refute skepticism, which claims that knowledge is not possible and that no objective truth exists.
• Sophist Gorgias is an example of a skeptic. He argued:
  1. Nothing exists.
  2. Even if something exists, we cannot know it.
  3. Even if we know it, we cannot communicate it.
• Skeptics say discussion is only for defending one’s own position, not for finding truth, which reduces philosophy to mere rhetoric.
• Plato gives two main arguments against skepticism.

1. Skepticism is Self-Contradictory

• If someone says “nothing is true,” then that statement itself cannot be true.
• If someone claims “knowledge is impossible,” they are already making a knowledge-claim.
• Therefore, skepticism defeats itself, just like relativism.

2. Certain Knowledge Exists (Mathematics & Geometry)

• Plato argues that there are some truths we know with certainty, especially in mathematics and geometry.
• In the dialogue Meno, Socrates helps a slave boy solve a geometrical problem, showing that clear reasoning leads to certain knowledge.
• Example: To create a square double the size of another square, draw a new square using the diagonal (DB) of the original square as the side.
• The first square has two equal triangles, while the new square has four of the same triangles — proving the new square is exactly double in area.
• This demonstrates that some knowledge is definite, so the claim “knowledge is impossible” is false.

Summary

Plato refutes skepticism by showing that the statement “knowledge is impossible” contradicts itself and by demonstrating that certain knowledge, especially in mathematics and geometry, is clearly possible. This allows Plato to move forward in building his theory of knowledge.

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Knowledge Is Not Sense-Perception or Opinion

• After proving that knowledge is possible, Plato moves to the second stage: explaining what knowledge is NOT.
• Before defining knowledge, Plato removes wrong assumptions.
• He clarifies that knowledge is not sense-perception and not mere opinion.
• Sense-perception means knowing through our senses (seeing, hearing, touching, etc.).
• Later, this idea becomes known as Empiricism, which claims that all knowledge comes only from the senses.
• Plato argues against this view.

Sense-Perception Cannot Be Knowledge

• Through senses, we only know the changing physical world.
• Heraclitus described this world as constant flux (always changing).
• Knowledge that changes cannot be true knowledge, because true knowledge should be stable and certain.
• Example:
  • If you say, “This tea is very hot,” it may be true now.
  • But after 10 minutes, it becomes cold, and the statement becomes false.
• Another example:
  • A flower looks beautiful today but may wither in a few days.
• Therefore, sense-based information is temporary and cannot be called real knowledge.

Summary

Plato explains that knowledge cannot come from sense-perception, because the physical world is always changing. Something that changes cannot be fully true, so real knowledge must come from a more stable and reliable source than the senses.

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Why Sense-Perception Cannot Be True Knowledge (Further Arguments)

• Plato’s second argument is that sense-based knowledge depends on the perceiver.
  One person may find the tea tasty, another may find it bad. So senses show only personal experience, not objective truth.
• If knowledge depends on personal feelings, it leads back to relativism (Protagoras’ view: “man is the measure of all things”), which Plato has already refuted.
• Therefore, through senses we only know how things appear to us, not what they really are.
  Appearance and reality are different.

Language and Changing Objects

• Plato’s third argument is that sense-perception cannot give knowledge because language needs stable meanings.
• Words represent things, but the things we perceive through senses are always changing.
  • Example: A flower today is fresh, tomorrow it will wither and fall.
• If a word like “flower” refers only to changing physical objects, then its meaning would also keep changing, making communication impossible.
• But language works because when we say “flower”, we refer to an unchanging universal idea of a flower, not a single temporary one.

Perception vs. Concept

• Plato explains the difference between what we perceive and the concept we think of.
• Example: When we draw a line, we call it a “line,” but the drawing is not a perfect line.
  • A true geometrical line has only length and no width.
  • The drawn line has width when examined closely.
• So the drawn line is only a representation, not the real concept of a line.
• This shows that concepts (ideas) are stable, while sense objects are imperfect and changing.

Summary

Plato argues that sense-perception cannot give real knowledge because senses show only personal experiences and changing objects. Language and thought require stable, unchanging concepts. Therefore, true knowledge must come from concepts (ideas) rather than sensory experience.

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Perception vs. Conception and the Reality of Universal Concepts

• Plato explains that true knowledge does not come from sense-perception, but from conception — mental understanding of universal ideas.
• Perception is sensory experience (seeing, tasting, touching). It shows us particular, changing objects.
• Conception is a mental and abstract process that helps us understand universal concepts like line, circle, beauty, justice, etc.
• When we see many beautiful things, we form the concept of beauty; when we see many kind actions, we form the concept of kindness.
• The objects we see (drawn lines, flowers, mountains) are only imperfect representations of universal ideas, which are stable and unchanging.
• Language works only because the words we use refer to universal concepts, not to temporary physical objects.
  Example: The word pen refers to the idea of a pen, not any one particular pen.
• Sophists argued that concepts like justice, love, truth, goodness are just words with no real existence because we cannot see or touch them.
• Plato responds: If justice is only a word, then moral judgments lose all meaning — there would be no difference between a saint and a criminal.
• Since this conclusion is absurd, justice must be real.
• Justice cannot be physical (no shape, size, or color), so it must be non-physical, objective, and real.

Summary

Plato distinguishes between perception and conception, showing that senses reveal only changing appearances, while the mind grasps stable universal concepts. He argues that values like justice must be real and non-physical; otherwise, moral judgments would be meaningless. Real knowledge therefore concerns universal, unchanging truths rather than sensory objects.

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Sense-Perception Is Limited and Cannot Give Complete Knowledge

• The earlier argument we discussed was a reductio ad absurdum: we assumed “justice is just a word,” and when that led to absurd conclusions, we accepted the opposite — justice is real and non-physical.
• Now Plato moves further and argues that sense-perception is not complete knowledge.

Sense-Perception Has Limits

• There are many things we cannot see with senses, such as conclusions in mathematics and geometry.
• When we say someone has a good character, we do not see character with our eyes. Character is understood mentally, not through senses.

Senses Can Be Deceptive

• Our senses sometimes give false impressions:
  • Railway tracks appear to meet in the distance, but we know they do not.
  • In heat, mirage (mrig-trishna) makes us see water where none exists.
• If senses can mislead us, they cannot be a reliable foundation for knowledge.

Perception Without Knowledge Is Possible

• If a Japanese person speaks to you in Japanese, you see and hear him, but you don’t understand.
• Here, perception happens but knowledge does not.

Knowledge Without Perception Is Possible

• You can think of a friend who is not physically present.
• You are not perceiving them with senses, yet you have knowledge of them.

Conclusion

• Since we can have perception without knowledge and knowledge without perception, the two are not the same.
• Therefore, sense-perception cannot be equal to knowledge.

Summary

Plato shows that sense-perception is limited, often deceptive, and different from knowledge. Knowledge can exist without perception and perception can occur without understanding. Thus, true knowledge cannot come from the senses and must come from a higher, conceptual understanding.

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Further Arguments Against Sense-Perception as Knowledge

• Plato gives a few additional arguments to show that sense-perception cannot be the source of true knowledge.
• One playful argument says: if you cover one eye, you are seeing and not seeing at the same time.
  This suggests perception is partial and unstable, so it cannot be equal to complete knowledge.

Comparison of Sensory Data Requires Something Beyond Senses

• We know that color and sound are different.
• But this knowledge cannot come from senses alone:
  • The eyes see color but cannot hear sound.
  • The ears hear sound but cannot see color.
• No single sense organ experiences both color and sound together.
• Yet we are able to compare them and judge that they are not the same.
• This means there is something beyond the senses, a higher faculty, that:
  • Collects data from different senses
  • Compares it
  • Forms a judgment
• So true knowledge must come from the mind, not the senses.

Key Point

• Sense-perception only gives fragments of experience.
• The mind organizes, compares, and understands—so knowledge is a mental/cognitive activity, not a sensory one.

Summary

Plato adds final arguments showing that senses provide incomplete and separate information. Since we can compare different sensory inputs (like color and sound) without any single sense doing so, a higher mental faculty must be involved. Therefore, true knowledge comes from the mind, not from the senses.

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Why Belief or Opinion Is Not Knowledge

• After rejecting sense-perception as knowledge, Plato now explains why belief (or opinion) also cannot be called knowledge.
• A belief can be true or false, but knowledge is always true.
  We believe something because we think it is true — but it may later turn out to be wrong.
• Example: You believe you saw someone in a coffee shop. Later you learn the person was in another city.
  You will say: “Yesterday I thought he was there, but today I know he was not.”
• So yesterday was belief, today is knowledge — because knowledge means certainty, while belief is just assumption.

Belief Can Accidentally Be True

• Even if a belief becomes true later, that does not automatically make it knowledge.
• If you say “I think it is raining in Delhi” without any reason, and it really is raining — this is not knowledge.
• It is simply a lucky guess or coincidence.
• Knowledge requires justification, not just truth.

Key Difference

• Belief (Opinion) = may be true or false, based on feeling or guessing.
• Knowledge = must be true and supported by reason.

Summary

Plato shows that belief is uncertain and can be wrong, while knowledge must always be true. Even a true belief is not knowledge unless it is supported by proper reasoning. This separates belief from real knowledge.

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Knowledge as Justified True Belief (JTB)

• Plato explains the difference between true belief and knowledge.
• A belief can be true or false, but knowledge is always true.
• In the dialogue Meno, Plato writes that true beliefs become knowledge when they are “anchored”.
• The term anchored means made stable, like how a ship is held in one place by dropping an anchor.
• Similarly, a belief becomes stable and fixed when it has justification — a proper reason or evidence supporting it.
• A true belief without justification is just a lucky guess.
• A true belief with justification becomes knowledge, because it cannot be easily changed.
• Sophists could influence and change opinions through rhetoric, but justified knowledge remains stable.
• Plato gives an example:
  • A blind man and a sighted man both believe they should turn back before a cliff.
  • The blind man’s belief is a guess (true but unjustified).
  • The sighted man has reason — he can see the cliff.
  • Therefore, only the sighted man has knowledge.
• So, knowledge = Justified True Belief (JTB).

Difference Between Belief and Knowledge

• Belief can be true or false, but knowledge is always true.
• Belief is often unstable and can change; knowledge is stable.
• Belief does not require justification; knowledge requires strong justification.

Summary

Plato says that a mere true belief is not enough to be called knowledge. Only when a belief is both true and supported by solid justification does it become stable and reliable knowledge. This is known as the JTB theory — Justified True Belief.

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The Object of Knowledge: Universals (Ideas / Forms)

• If we say we have knowledge, it must always be knowledge of something — knowledge needs an object.
• That object of knowledge must be:
  • Objective (not based on personal feelings or opinions)
  • Universal (true for everyone, not just one person)
  • Beyond the senses (since sense-perception cannot give knowledge)
  • Unchanging (because knowledge is stable, so its object must also be stable)

Particulars vs. Universals

• The world we perceive through senses contains particulars — individual things that are constantly changing.
• But behind these changing particulars, Plato says there exists something unchanging.
• Example:
  • Ram, Shyam, Mohan, Sohan are all different individuals.
  • Yet, they all share a common property: Humanness.
  • This shared and unchanging property is what Plato calls a universal.
• More examples:
  • Many beautiful objects exist (flower, painting, music), but Beauty itself does not change.
  • Many rectangular objects exist (book, mobile, screen), but Rectangularity is constant.

Universals Are Stable

• Particulars change — they grow, decay, and disappear.
• Universals do not change — they remain the same in all cases.

Plato’s Names for Universals

• These unchanging realities are called:
  • Universals
  • Ideas
  • Forms
• The forms are the true objects of knowledge because they are objective, stable, universal, and non-physical.

Summary

Plato concludes that real knowledge must be about something unchanging and universal. The changing physical world cannot be the source of knowledge. Therefore, knowledge is about Forms or Universals — the stable realities behind all changing particulars. These will be studied in detail in the next lecture on Plato’s metaphysics.

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How We Gain Knowledge: Recollection and Dialectic

• Knowledge has an object (Forms), but Plato next explains how we come to know those Forms.
• In the dialogue Meno, a paradox is raised:
  • If you already know something, you don’t need to search for it.
  • If you don’t know it, you won’t recognize it even if you find it.
  • So, learning seems impossible.
• Plato answers that we both know and don’t know universal Forms.
• We know them because the soul already possessed this knowledge before entering the body.
• When the soul enters the body, it forgets this knowledge — so in life we are not aware that we know.
• Therefore, learning is not gaining new knowledge, but recollection — remembering what the soul already knows.
• This view is called innate knowledge: knowledge is in us from before birth.
• The process of bringing out this forgotten knowledge is called Dialectic.
• Dialectic is the Socratic method of questioning, through which hidden knowledge in the mind becomes clear.
• In Meno, Socrates uses step-by-step questions to guide a slave boy to solve a geometry problem, proving that knowledge is recollection.

Summary

Plato explains that we already possess knowledge of universal Forms in the soul, but we forget it when we enter the body. Learning is therefore a process of recollecting this hidden knowledge through dialectic, the method of careful questioning and reasoning.

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Conclusion of Plato’s Epistemology

• In this lecture, we completed Plato’s theory of knowledge (epistemology).
• We learned:
  • Knowledge is possible, and relativism and skepticism are false.
  • Knowledge is not sense-perception, because senses deal with the changing world.
  • Knowledge is not mere belief, because belief can be false or unstable.
  • Knowledge = Justified True Belief (JTB) — a belief must be both true and supported by reason to count as knowledge.
• The object of knowledge is something objective, universal, unchanging, and beyond the senses — called Forms / Ideas / Universals.
• In the next lecture, we will study Plato’s metaphysics, where we explore what Forms are and how they exist.
• The speaker emphasizes that studying philosophy requires continuity, because every concept builds on the previous one.
• Philosophy is not motivation or history; it is a deep, structured discipline requiring patience and step-by-step understanding.
• Future study will include:
  • Various types of logic
  • Key philosophical concepts like necessity, contingency, causality
  • Application of philosophy to real problems in science, ethics, language, and reasoning.
• Terms like skepticism, relativism, dialectic, monism, JTB, sense-perception, etc., will continue to be used, so no lecture should be skipped.

Summary

This section concludes Plato’s epistemology. We now understand what knowledge is, what it is not, and why its object must be universal Forms. The next lectures will move into Plato’s metaphysics, where these Forms are studied in detail. Students are encouraged to follow the series consistently, as philosophical learning is cumulative and interconnected.