text: Among the most difficult problems in knowledge representation are the breadth of commonsense 
  knowledge (the set of atomic facts that the average person knows is enormous);[29] and the sub-symbolic 
  form of most commonsense knowledge (much of what people know is not represented as "facts" or 
  "statements" that they could express verbally).[16] There is also the difficulty of knowledge acquisition, 
  the problem of obtaining knowledge for AI applications.[c] 