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Plato on Knowledge: Understanding Justified True Belief

Updated on September 16, 2011

The definition of knowledge has long plagued philosophers. Plato, founder of Western philosophy, tackled this very question around 400 B.C. According to Plato's philosophy, in order to have Knowledge, one must also have Justified True Belief. Each of these terms, for Plato, are necessary for the existence of knowledge.

Knowledge vs. Belief

In order to understand Plato's vision of knowledge, we must first make a concrete distinction between knowledge and belief. While its easy to say that opinion is not the same as knowledge, especially when it comes to opposing political views, this gets a bit trickier when it comes to belief. Essentially, a belief is akin to an opinion. It is one person's viewpoint that is not necessarily grounded in any facts. A person could believe that the earth revolves around the moon, or that the tooth fairy does indeed make a visit underneath every kid's pillow. The person could even believe that these are facts. Yet without a means to prove these "facts" as more than hypothesis, they remain opinion or belief, rather than knowledge.

Essentially beliefs can be defined as assumptions that we have formed about the world around us. Even a belief that happens to be true does not automatically become knowledge on the basis that it is fact. For example, a contestant on a game show, when asked, "Which president was taller, Abe Lincoln or Grover Cleveland?" guesses Lincoln. Every picture that the contestant has ever seen of Lincoln has showed him to be an extremely tall man. The contestant also cannot remember ever seeing a picture of Cleveland. It would be the opinion of the contestant that Lincoln was likely taller, yet he probably wouldn't bet a million dollars on it.

Because Lincoln actually was the tallest of all presidents, it would turn out that the contestant's opinion was correct, or true. However, just because he had a right opinion, did not mean that he knew that information. Thus even when the opinion or belief is correct, it cannot be considered actual knowledge.

Can you win the million with truth alone?
Can you win the million with truth alone?

True Belief

Though belief and knowledge have been defined as two separate entities, it does not mean that in the Platonian view there can be one without the other. If I was to say that I know one plus one equals two, it would make no sense to say that I know this fact without believing it. Within Plato's formula, knowledge implies belief inherently.

Just as one must have a belief to have knowledge, it is also a requirement that it be a true belief. One cannot know something that is untrue. It would be impossible to find a person that could that they know that one plus one equals five, and there is not nor will ever be any sort of evidence that could support this statement as true.


The final component of knowledge, according to Plato, is justification. Without justification, says Plato, all we have is simply true opinion. Plato calls this justification a "tether," and uses an analogy of statues that will run away if not tied down. While a slightly esoteric example, what Plato is implying is that true opinion is fleeting. Belief is a state of mind, which can often be fickle and liable to change.

For example, let's say our game show contestant, who had seen many pictures of a tall Lincoln, suddenly remembered that in all those pictures Lincoln was wearing a hat. In light of this memory, the contestant might start to waver in his conviction that Lincoln was the taller president. Furthermore, if the contestant remembered an old college history professor once mentioning that Cleveland was a very tall man, he could even change his mind completely and opt for Cleveland as the safer answer.

Justification is the factual rationalization of true opinion, the thing that grounds it in reality. Without this justification, the true belief that Lincoln is the taller man is nothing more than a lucky guess or hypothesis. Similarly, if the contestant were to answer Cleveland, he would still only be relying on belief. What is lacking is the reliable information that places the entity of Lincoln as taller than the entity of Cleveland. This would be the contestant's rationale, the way that he could actually justify his belief that Lincoln was the taller man. If this justification were present, the contestant would likely then be willing to go ahead and bet the million on the question! The contestant would be able to say, "I know Lincoln is taller," because:

  1. The statement is correct (True).
  2. He believes that it is correct (Belief).
  3. He is justified in believing that it is correct (Justified).

According to Plato, our contestant would be the proud possessor of Knowledge.


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    • profile image


      8 years ago

      he is really the foundation

    • Silver Poet profile image

      Silver Poet 

      8 years ago from the computer of a midwestern American writer

      I like Plato, Socrates, Aristotle, Ben Franklin, and others who think. Thanks for the thoughtful hub!

    • profile image

      Roner M. Anildes 

      8 years ago

      Well done!

    • Anaya M. Baker profile imageAUTHOR

      Anaya M. Baker 

      9 years ago from North Carolina


      Accusing a person of plagiarism is a very serious accusation, and should not be taken lightly. In an academic setting, plagiarism, even unintentional, leads to expulsion for a student or termination for a faculty member. In the writing world, a person can be banned from a site. If you want to accuse someone of plagiarism, it is important to have some evidence to back up your statement.

      As for my own article, (and anything else I publish) I stand behind everything I wrote. It is all original content.

      As an English student, I am constantly running across philosophic theories embedded in literature. Because I did not study philosophy in school, I've had to some research to unravel just what the authors are talking about. In my struggle to understand Plato's theory of knowledge, I came up with the analogy of the game show contestant. After that, it made sense. I decided to write a hub on the theory in hopes that what worked for me might be useful to someone else. Since I don't make any money on these types of hubs, my only motivation is knowledge sharing. If its useful to someone, great! If not, don't bother reading.

      Since my article was in no way plagiarized, I can only assume you are calling me "a useless person crying for people to validate me as an intellectual." It seems that my writing/topic didn't meet your standards. Perhaps next time you could exercise your option of choice and not bother reading articles that you don't like.

      To all those who left such kind and supportive comments, it is much appreciated, especially after this, my first instance of hate-mail. I'm not an expert by any means, just working on getting a basic knowledge to better understand some of my favorite writers. I'm hoping to post some more philosophy-related articles here on's fun!

    • profile image


      9 years ago

      Unoriginal, plagerized, verbal diarrhea. If you're going to plagiarize, at least pick something different from useless people crying for us to validate them as intellectuals.

    • profile image

      Moses Sense Simukonde 

      9 years ago

      Knowledge is the Result of experience, and understanding of the phenomena and judgement. but i can also that not all knowledge comes to us in this way. some ways that humans come know things have no explanation what so ever yet. people may speculate but who knows if what they say is concrete. in any case absolute scepticism is imposible too.

    • epigramman profile image


      9 years ago

      ...well the 'knowledge' I have upon entering your hallowed hubspace is based on this firm 'belief' - you have certainly engaged my mind in some of the most interesting subjects to grace hubpages in both an enlightening and entertaining mannner .......

    • sligobay profile image


      9 years ago from east of the equator

      Skidmore grounded you quite well and your understanding and communicative skills are apparent. I have just completed a reread of Georgias, one of Plato's Socratic dialogues and Plato himself would disagree that he is the father of Philosophy. He would credit Socrates with that laurel.

      In the final monologue of Socrates, he accepts "as fact" that there is a final judgement at death for all men. He had faith in this element of Greek Mythology. This fact cannot be proven and is therefore belief rather than knowledge. It cannot be justified.

      I thought this aspect interesting as a demonstration that reason and belief can coexist and have continued to coexist since the advent of philosophy in ancient Greece.

      Congratulations on your success in describing a complex issue with simplicity and clarity. This old Colgate grad is now gladly following.

    • Nell Rose profile image

      Nell Rose 

      9 years ago from England

      HI, very interesting, I do believe the world is based on belief more than knowledge. Knowledge is a fact as much as can be proved, faith in something is usually a case of passed down ideas culminating in a false truth, thanks nell

    • Paladin_ profile image


      9 years ago from Michigan, USA

      Very well written, with consistent examples. Well done!

    • My Esoteric profile image

      Scott Belford 

      9 years ago from Keystone Heights, FL

      I suspect this is an area where Plato and Aristotle would not disagree, although Aristitle might quible about the Justification piece in some circumstances. Very nice hub Anaya. Another where it is needed and is very clear to me is in the question "Will the sun rise tomorrow?" The answer today is yes, as it meets all three of Plato's criteria, especially the third, since today we all know that one day, sometime in the distant future, the sun will not rise tomorrow.

      Question for you: How do you think Plato would handle an optical illusion such as the Ponzo Illusion. This is where you have two parallel lines (say railroad tracks) converging in the distance via perspective and two equally long lines drawn on the picture across the tracks, one in the foreground and one in the background. Most people will swear the line in the back is longer than the line in the front.

      By the way Vrbmft, I wish I had majored in Philosophy! At 63, I am thinking about doing it anyway. I have been studying it, via The Teaching Company, for a year or so and found that instead of it being something for people with square heads wearing round pants to do, it is actually the foundation of all current society and science today :-).

    • satomko profile image

      Seth Tomko 

      9 years ago from Macon, GA

      Plato is always welcome. Thanks for this hub.

    • dahoglund profile image

      Don A. Hoglund 

      9 years ago from Wisconsin Rapids

      Aah, if only this were taught in our schools. I get so frustrated watching politics and illogicality of most of it. Rather than seek truth, too often they try to bend truth to fit preconceived bias.I vote this uo, beautiful and useful.

    • vrbmft profile image

      Vernon Bradley 

      9 years ago from Yucaipa, California

      Hello Anaya

      You are a brave soul to tackle philosophy and you tackle it well. Very clearly presented and interesting as well.

      I majored in philosophy and have a BA in philosophy! Big Whoopie, right? But I love philosophy and it has helped me see beyond what my eyes see and hear what words are really saying and so on and so on. Also helps me PAY ATTENTION. An interesting concept from the Peaceful Warrior.

      Anywho, tackle some more, Anaya. Good stuff. I want to encourage you! I will read. And there is also an interesting distinction between one's beliefs and one's values. Beliefs are beliefs, and values are the demonstration of those beliefs in our behavior. Looking forward to more philosophical hubs from you.



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