Abstract
Our whole life (attitudes, reactions, actions) is based on knowledge. Depending on our present state of knowledge, we react to different situations in different ways, give different answers to different questions. To do that, one has to know something. Even though we do not tend to question our knowledge in everyday life, it is interesting to do so in order to (ideally) find an answer to the sceptics who deny that knowledge exists, who assert that we cannot know anything for sure. In this paper, I will address the question: "How (if at all) do you know that you are not a brain in a vat? which is a question posed by the skeptics who intend to show that one cannot know the answer to it.
