text: **Knowledge representation**
  Knowledge representation and knowledge engineering[17] 
  allow AI programs to answer questions intelligently and 
  make deductions about real-world facts. Formal knowledge 
  representations are used in content-based indexing and 
  retrieval,[18] scene 
  interpretation,[19] clinical decision 
  support,[20] knowledge discovery (mining "interesting" and 
  actionable inferences from large databases),[21] and other 
  areas.[22] 
  
  A knowledge base is a body of knowledge represented in a 
  form that can be used by a program. An ontology is the set 
  of objects, relations, concepts, and properties used by a 
  particular domain of knowledge.[23] Knowledge bases need 
  to represent things such as objects, properties, categories, 
  and relations between objects;[24] situations, events, states, 
  and time;[25] causes and effects;[26] knowledge about 
  knowledge (what we know about what other people 
  know);[27] default reasoning (things that humans assume are true until they are told differently and will 
  remain true even when other facts are changing);[28] and many other aspects and domains of knowledge. 
  
  An ontology represents knowledge as a set 
  of concepts within a domain and the 
  relationships between those concepts. 