text: In most languages using the CWA we assume that everything that is currently known about the system is 
  already in the database. However, OWL was meant to be a language to bring semantics to the Internet so 
  the language designers chose the OWA. The open world assumption means that we cannot assume 
  something doesn’t exist just because it isn’t currently in the ontology. The Internet is an open system. The 
  information could be out there in some data source that hasn’t yet been integrated into our ontology. 
  Thus, we can’t conclude some information doesn’t exist unless it is*explicitly stated that it does not exist*. 
  In other words, because something hasn’t been stated to be true, it cannot be assumed to be false — it is 
  assumed that the knowledge just hasn’t been added to the knowledge base. In the case of our pizza 
  ontology, we have stated that MargheritaPizza has toppings that are kinds of MozzarellaTopping 
  and also kinds of TomatoTopping. Because of the open world assumption, until we explicitly say that a 
  MargheritaPizza only has these kinds of toppings, it is assumed by the reasoner that a 
  MargheritaPizza could have other toppings. To specify explicitly that a MargheritaPizza has 
  toppings that are kinds of MozzarellaTopping or kinds of TomatoTopping and only kinds of 
  MozzarellaTopping or TomatoTopping, we must add what is known as a closure axiom on the 
  hasTopping property. 