Gettier problem

The Gettier problem is a problem in the field of philosophy called "epistemology". Epistemology deals with how we know things. The Gettier problem is a problem which explains that sometimes, people can think they know something, but actually they don't. The problem is named after a philosopher called Edmund Gettier, who wrote a paper about it.

Imagine you want to know if your friend has a cat. Your friend tells you that he does, and you believe him. However, it turns out that your friend is wrong - he doesn't have a cat at all. In this case, even though your friend told you something, you still don't know whether or not your friend has a cat! That's the Gettier problem - even though you think you know something, it turns out you don't.