The problem in any kind of existence begins from a very old distinction between appearance and reality. Appearances are obviously how things seem to us in our perception although not everything that we perceive does really also exist. How things seem to us is a property of our perceptual apparatus—senses, mind, brain, etc. Reality, on the other hand, is supposed to be independent of this perceptual apparatus. Therefore, how do we know that what appears to us is also real? Could it be that we are hallucinating or dreaming and what appears to us does not in fact exist?