Two methods of inference often are used, forward and backward chaining. Forward chaining is a top down method which takes facts from satisfied conditions in rules which lead to actions being executed. Backward chaining is the reverse. It is a bottom up procedure which starts with goals (or actions) and queries the user about information which may satisfy the conditions contained in the rules .It is a verification process rather than an explorationprocess. An example of backward chaining is MYCIN(VMS81), and an example of forward chaining is EXPERT(WK81). A system which uses both is Prospector
(DGH79)