The form of a modus tollens argument resembles a syllogism, with two premises and a conclusion: The first premise is a conditionalclaim, such as P implies Q. The second premise is an assertion that Q, the consequent of the conditional claim, is not the case. From these two premises it can be logically concluded that P, the antecedent of the conditional claim, is also not the case. For example: Supposing that the premises are both true, it follows that no intruder has been detected. This is a valid argument since it is not possible for the conclusion to be false if the premises are true. Another example: Another example:
Relation to ''modus ponens''
Every use of modus tollens can be converted to a use of modus ponens and one use of transposition to the premise which is a material implication. For example: Likewise, every use of modus ponens can be converted to a use of modus tollens and transposition.
Formal notation
The modus tollens rule can be stated formally as: where stands for the statement "P implies Q". stands for "it is not the case that Q". Then, whenever "" and "" each appear by themselves as a line of a proof, then "" can validly be placed on a subsequent line. The modus tollens rule may be written in sequent notation: where is a metalogical symbol meaning that is a syntactic consequence of and in some logical system; or as the statement of a functional tautology or theorem of propositional logic: where and are propositions expressed in some formal system; or including assumptions: though since the rule does not change the set of assumptions, this is not strictly necessary. More complex rewritings involving modus tollens are often seen, for instance in set theory: Also in first-order predicate logic: Strictly speaking these are not instances of modus tollens, but they may be derived from modus tollens using a few extra steps.
The validity of modus tollens can be clearly demonstrated through a truth table.
p
q
p → q
T
T
T
T
F
F
F
T
T
F
F
T
In instances of modus tollens we assume as premises that p → q is true and q is false. There is only one line of the truth table—the fourth line—which satisfies these two conditions. In this line, p is false. Therefore, in every instance in which p → q is true and q is false, p must also be false.
Modus tollens represents an instance of the law of total probability combined with Bayes' theorem expressed as: where the conditionals and are obtained with Bayes' theorem expressed as: and . In the equations above denotes the probability of, and denotes the base rate of. The conditional probability generalizes the logical statement, i.e. in addition to assigning TRUE or FALSE we can also assign any probability to the statement. Assume that is equivalent to being TRUE, and that is equivalent to being FALSE. It is then easy to see that when and. This is because so that in the last equation. Therefore, the product terms in the first equation always have a zero factor so that which is equivalent to being FALSE. Hence, the law of total probability combined with Bayes' theorem represents a generalization of modus tollens.
Subjective logic
Modus tollens represents an instance of the abduction operator in subjective logic expressed as: where denotes the subjective opinion about, and denotes a pair of binomial conditional opinions, as expressed by source. The parameter denotes the base rate of. The abduced marginal opinion on is denoted. The conditional opinion generalizes the logical statement, i.e. in addition to assigning TRUE or FALSE the source can assign any subjective opinion to the statement. The case where is an absolute TRUE opinion is equivalent to source saying that is TRUE, and the case where is an absolute FALSE opinion is equivalent to source saying that is FALSE. The abduction operator of subjective logic produces an absolute FALSE abduced opinion when the conditional opinion is absolute TRUE and the consequent opinion is absolute FALSE. Hence, subjective logic abduction represents a generalization of both modus tollens and of the Law of total probability combined with Bayes' theorem.
