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material conditional (material implication) implies, if P then Q, it is not the case that P and not Q propositional logic, Boolean algebra, Heyting algebra: is false when A is true and B is false but true otherwise.
With Peirce's law, one can add (at no cost) additional premises of the form Z→P to Γ. For example, suppose we are given P→Z and (P→Q)→Z and we wish to deduce Z so that we can use the deduction theorem to conclude that (P→Z)→(((P→Q)→Z)→Z) is a theorem. Then we can add another premise Z→Q. From that and P→Z, we get P→Q.
Then we have by the law of excluded middle [clarification needed] (i.e. either must be true, or must not be true). Subsequently, since P → Q {\displaystyle P\to Q} , P {\displaystyle P} can be replaced by Q {\displaystyle Q} in the statement, and thus it follows that ¬ P ∨ Q {\displaystyle \neg P\lor Q} (i.e. either Q {\displaystyle Q ...
The proposition to be proved is P. We assume P to be false, i.e., we assume ¬P. It is then shown that ¬P implies falsehood. This is typically accomplished by deriving two mutually contradictory assertions, Q and ¬Q, and appealing to the law of noncontradiction. Since assuming P to be false leads to a contradiction, it is concluded that P is ...
For example, even though material conditionals with false antecedents are vacuously true, the natural language statement "If 8 is odd, then 3 is prime" is typically judged false. Similarly, any material conditional with a true consequent is itself true, but speakers typically reject sentences such as "If I have a penny in my pocket, then Paris ...
In other words, a logical truth is a statement which is not only true, but one which is true under all interpretations of its logical components (other than its logical constants). Thus, logical truths such as "if p, then p" can be considered tautologies.
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P, as an individual or a class, materially implicates Q, but the relation of Q to P is such that the converse proposition "If Q, then P" does not necessarily have sufficient condition. The rule of inference for sufficient condition is modus ponens , which is an argument for conditional implication: