Justified True Belief

What constitutes knowledge? According to one school of thought, knowledge can be defined as Justified True Belief (JTB). In this blog post, we’ll go over a few examples of JTB as well as discuss arguments against the definition.

Under the JTB theory, knowledge must be believed (B), true (T), and justified(J). In order to be able to say “I know X”, then you must validate that you can say “I believe X, X is true, and I am justified in believing X.”


What does it mean to believe something? According to JTB, you can only know what you believe. If I don’t believe the earth is round, then I cannot know that the earth is round (even if it is true the earth is round). It’s important to recognize that knowledge is not truth. Knowledge is a particular relation to the knower, and for the knower to know, the knower must believe that it is true.


Truth is a word that comes with a lot of baggage. In context of JTB, let’s consider truth as how things are versus how things are shown to be. For example, a spy gives the government false information about a foreign country. This false information is opposed by the actual, true information that the spy withheld. The government assumes the information given is the true information. Therefore, truth in this case, is the expected result of some intentional act. If the government believes the information to be true, but it actually isn’t true, then the government does not know the actual information.


In order for a belief to be justified, it must be grounded in some epistemic argument. For example, if I simply believe the earth is round, but have no proof to show why I hold that belief, then (even if it’s true), I don’t know it. So, what sort of justification is required? The justification needs to be appropriate. But then what does it mean to be appropriate? This is where things get a bit confused. Appropriateness is something that can be both subjectively and collectively given. If I, myself, find the evidence appropriate, then my conscience won’t demand further evidence. If I am attempting to prove to a large community that I am justified, then I will need evidence that is appropriate to the standards of the community.

Examples of Justified True Belief

Photo by Joe Caione on Unsplash

Dog in the park: I am walking through a park. I believe a dog is in the park, but a doubt invades my conscious. I don’t know it yet. Independent of my believing, a dog is on the other end of the park. I still don’t know it yet. Finally, I walk towards the dog, see it with my eyes, and verify it is in fact a dog. Now I know it because I am justified through my sight that a dog is there in the park.

Photo by Jozsef Hocza on Unsplash

The keys are in my pocket: I rush out the door and believe my keys are in my pocket. I feel myself justified that I have my keys because I have never before forgotten my keys. But I still don’t know it yet. I reach into my pocket and find my keys are not there. At no point did I have knowledge that keys were in my pocket.

Photo by Michael D Beckwith on Unsplash

Legislation will improve education: As a legislator, I propose legislation that I believe will improve education. A group of members argue I am not justified because similar legislation was passed last year, and it didn’t improve education. Nonetheless, the bill gets passed. Education over the year improves, thus I know that the new legislation improved education. However, the group of members are still not convinced that we are justified in believing that the new legislation resulted in the improved education. I know it, but the group of members do not know it.

Arguments against Justified True Belief

Some make the claim that JTB is not an adequate definition for knowledge. Most notably of these cases come from Edmund Gettier. In his 1963 paper, “Is Justified True Belief Knowledge?” he proposes a new case which meets the criteria for JTB, but is not actual knowledge. Let’s assume a couple cases:

Car Tank. Let’s assume I’m driving a car, and I believe that I have a full tank of gas in the tank. My belief is justified by the gauge which shows a full-bar. In actuality, the gas tank is actually full. Therefore, it meets the JTB case, and should be considered knowledge. However, the gauge I use to justify my belief is actually broken and always shows a full-bar. It just happens to be the lucky case that the tank of gas was full that my belief seems to be true.

Broken Clock. I believe it to be 1pm. I walk up to a clock, and it say’s 1pm. But the clock is actually broken, and it always reads 1pm (and I don’t know this). But indeed, it actually is 1pm. This meets JTB, but it is not actual knowledge according to Gettier.

It seems in these cases, it amounts to getting “lucky” that the method of justification is not reliable, but it happens to be read in a way that adheres to the actual true. To get around this problem, we can either expand the JTB to include such “luck” cases.



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Rob McQueen

Rob McQueen

Philosophy and Software Engineering