I’ve put a lot of thought into this question once given the prompt, and more and more I thought about it, there were less and less actions/object that I knew for certain, and the term “know” was too vague to make judgements off of to what I surely judge if anything fit the definition of “knowing.” But lets start at the bottom. Assume that I doubt everything. What can we truly know for certain? In this viewpoint, you cannot know if the world, your country, or you, as an individual, exist. But as mentioned before, you are able to doubt, which means that you are currently in the process of thinking. So even if everything is a dream, and everyone else is fictional, at least you know that you are indeed thinking. So this explains that if one is doubting one’s existence, it proves that one’s mind is real to some degree; there must be a thinking entity (you) for there to be thought. This is where the famous saying “I think, therefore, I am” or “Cogito, ergo sum” comes from. So even if you doubt everything, this is the baseline you come to face with when defining what you know for sure. Even if we say that we can never know anything for certain, we know for certain that we exist.