 There are other metaphysical systems that make the existence of some concrete entities necessary without implying that there are any necessarily existing concrete things. In his Tractatus phase, Ludwig Wittgenstein takes a world to be a totality of facts. A fact consists of one or more objects related to each other in a certain way. By an act of selective attention, we concentrate on just the objects or just the relations. But objects and relations are always inextricably bound up with each other. Since every fact requires at least one object, a world without objects would be a world without facts. But a factless world is a contradiction in terms. Therefore, the empty world is impossible.