'What is real?' has been one of the key questions of philosophy since its beginning in antiquity. It is a question that, due to such films as The Matrix, has also made its way into popular culture. But it is not just a question philosophers ask. It is also asked by scientists when they investigate whether the fundamental constituents of matter are actually 'out there' or just a mere abstraction from a successful theory. Cognitive scientists ask it when trying to find out which set of the bewildering array of data processed by our brain could constitute the basis for such supposedly fundamental entities like the free agent or the self. This Very Short Introduction discusses what reality is by looking at a variety of arguments, theories and thought-experiments from philosophy, physics, and cognitive science. ABOUT THE SERIES: The Very Short Introductions series from Oxford University Press contains hundreds of titles in almost every subject area. These pocket-sized books are the perfect way to get ahead in a new subject quickly. Our expert authors combine facts, analysis, perspective, new ideas, and enthusiasm to make interesting and challenging topics highly readable.