Phenomenology
Martin Heidegger, a famous phenomenologist, said that philosophers should be looking at how we live in our "average everydayness."  In his book Being and Time, he uses many examples of how people live in their normal lives. Jean-Paul Sartre did the same in his book Being and Nothingness. For example, he writes about meeting a friend at a cafe. However, the friend, named Pierre, never shows up. Sartre writes that he can feel the absence of Pierre in the cafe. Because of this, he says that non-Pierre (or the lack of Pierre) is something that exists, at least for him. It exists because he can feel it.