Modus non excipiens - Misplaced Pages

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Transformation rules
Propositional calculus
Rules of inference
Implication introduction / elimination (modus ponens) Biconditional introduction / elimination Conjunction introduction / elimination Disjunction introduction / elimination Disjunctive / hypothetical syllogism Constructive / destructive dilemma Absorption / modus tollens / modus ponendo tollens Negation introduction
Rules of replacement
Associativity Commutativity Distributivity Double negation De Morgan's laws Transposition Material implication Exportation Tautology
Predicate logic
Rules of inference
Universal generalization / instantiation Existential generalization / instantiation
v t e

In logic, modus non excipiens is a valid rule of inference that is closely related to modus ponens. This argument form was created by Bart Verheij to address certain arguments which are types of modus ponens arguments, but must be considered to be invalid. An instance of a particular modus ponens type argument is

A large majority accept A as true. Therefore, there exists a presumption in favor of A.

However, this is an argumentum ad populum, and is not deductively valid. The problem can be addressed by drawing a distinction between two types of inference identified by Verheij:

Modus ponens:

Premises:

As a rule, if P then Q
P

Conclusion:

and

Modus non excipiens

Premises:

As a rule, if P then Q
P
It is not the case that there is an exception to the rule that if P then Q

Conclusion:

References

Bart Verheij, ’Logic, Context and Valid Inference Or: Can There be a Logic of Law’, 2000. Available on bart.verheij@metajur.unimaas.nl,
Walton, Douglas; Are Some Modus Ponens Arguments Deductively Invalid?, University of Winnipeg

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