Explaining the Gettier Problem

Wyatt Sell
4 min readOct 2, 2022

Philosophy is a subject that intimidates me (and many others), due to its tendency to use words that many don’t fully understand and many concepts requiring the reading of vast volumes to completely grasp. The Gettier Problem, however, is a shockingly accessible epistemological (the branch of philosophy concerned with knowledge) problem that is considered, to quote Wikipedia “a landmark philosophical problem”.

The problem in question is put forward by Edmund Gettier in a brief 3-page paper titled “Is Justified True Belief Knowledge?” (Full text), The crux of the paper lies with the refutal of the following definition of someone “knowing” a given proposition — which is commonly called “Justified True Belief” (JTB).

For instance, let’s suppose that my brother is watching a movie downstairs. What are the conditions for me “knowing” this is true?

The three elements of justified true belief

This definition is credited with being first proposed by Plato (although he later argues against it). The conditions described may seem reasonable, but they fall foul of several scenarios put forth by Gettier:

Scenario 1

I’m going to adapt the movie example above throughout, but I’d encourage anyone to read the original paper to see the original scenarios.

Gettier’s “Scenario 1”

Now from this example, we can see if it fulfils the conditions put forth in our original definition of JTB:

  1. It is true: The person watching the movie does indeed have green hair
  2. I believe it’s true: I believe that my brother, who has green hair, is watching the movie, and hence I believe that the person watching the movie has green hair
  3. I have evidence that its true: My brother, who has green hair, told me he’s going to watch a movie. ✅

We can see that this scenario fulfils all the relevant conditions for JTB, however, Gettier argues one can equally see that I do not know that the person watching the movie has green hair, given that my belief is based on an mistruth:

  • My brother is not in fact watching the movie, which is the core evidence my belief is based upon
  • My belief is true by virtue of there being a green-haired intruder watching the movie — something I don’t know

Scenario 2

This scenario is somewhat more nuanced than the previous one, so take a moment to work it through in your head.

Gettier’s “Scenario 2”

I can combine (1) and (2) to create a scenario which combines both — I have evidence for (1), and hence when creating the Either (1) OR (2) scenario, I am inferring it is true, due to my evidence for (1). In other works I’m not expecting (2) to be true, but by virtue of my evidence for (1), I can say that I’m justified in believing the Either (1) OR (2) expression.

Testing this against the original JTB definition:

  1. It is true: My sister is in Paris, fulfilling the scenario ✅
  2. I believe it’s true: I believe my brother is watching the movie, hence, I believe that the Either OR statement is true. ✅
  3. I have evidence that it’s true: My brother told me he’s going to watch a movie, and I heard the movie playing, therefore I have evidence that the Either OR expression is true. ✅

Now, as shown in the “illustration” (I use that word lightly), it just so happens that my brother is not watching the movie, and, by complete coincidence, my sister is in Paris. I don’t therefore “know” that this scenario is true, even though, it fulfils the above conditions.

Gettier uses these two counter-examples to successfully argue that the accepted definition of JTB is inadequate and it was met with much fanfare. Attempts have been made to adjust our definition by which someone can “know” something, but they fall beyond the scope of this article. You can read more about responses to the paper here:

Here’s the full text, which I highly recommend reading:

There’s been many people that have written or otherwise produced explanations of the Gettier problem, and I’d encourage anyone with more interest to seek them out — this is simply something I found interesting and I hope you enjoyed reading my article!