text: what role they play in inferences. One rule of inference states that, if something is necessary, then it is 
  . Another principle states that if a proposition is 
  also possible. This means that 
  .[128] 
  necessary then its negation is impossible and vice versa. This means that 
  
   follows from 
  
   is equivalent to 
  
  Other forms of modal logic introduce similar symbols but associate different meanings with them to 
  apply modal logic to other fields. For example, deontic logic concerns the field of ethics and introduces 
  symbols to express the ideas of obligation and permission, i.e. to describe whether an agent has to 
  perform a certain action or is allowed to perform it.[129] The modal operators in temporal modal logic 
  articulate temporal relations. They can be used to express, for example, that something happened at one 
  time or that something is happening all the time.[129] In epistemology, epistemic modal logic is used to 
  represent the ideas of knowing something in contrast to merely believing it to be the case.[130] 