The authors investigate a logical operator contained in a rule of inference as an unknown operator. It is shown that the function of the operator can be partially determined by solving the equation obtained from the rule of inference. The function can also be completely determined by this rule and an axiom system applied. This method makes clear the way in which a rule of inference restricts the function of an 'unknown' operator contained in the rule. The relative strength of certain inference rules is examined. Similarly, any other rule of inference can be investigated with this method. >
['Masamitu Otake', 'Taketoshi Mishima', 'Yoichi Manome']
Strength of the rule of inference