text: **Modal logic**
  
  Modal logic is an extension of classical logic. In its original form, sometimes called "alethic modal 
   expresses that 
  logic", it introduces two new symbols: 
  something is necessary.[126] For example, if the formula 
   stands for the sentence "Socrates is a 
   articulates the sentence "It is possible that Socrates is a banker".[127] To 
  banker" then the formula 
  include these symbols in the logical formalism, modal logic introduces new rules of inference that govern 
  
   expresses that something is possible while 