Contraposition

Article summary

In logic and mathematics, contraposition, or transposition, refers to the inference of going from a conditional statement into its logically equivalent contrapositive, and an associated proof method known as . The contrapositive of a statement has its antecedent and consequent negated and swapped. Conditional statement P \rightarrow Q . In formulas: the contrapositive of P \rightarrow Q is \neg Q \rightarrow \neg P . If P, Then Q. — If not Q, Then not P. "If it is raining, then I wear my coat." — "If I don't wear my coat, then it isn't raining." The law of contraposition says that a conditional statement is true if, and only if, its contrapositive is true. Contraposition ( \neg Q \rightarrow \neg P ) can be compared with three other operations: ;Inversion (the inverse), \neg P \rightarrow \neg Q :"If it is not raining, then I don't wear my coat." Unlike the contrapositive, the inverse's truth value is not at all dependent on whether or not the original proposition was true, as evidenced here. ;Conversion (the converse), Q \rightarrow P :"If I wear my coat, then it is raining." The converse is actually the contrapositive of the inverse, and so always has the same truth value as the inverse (which as stated earlier does not always share the same truth value as that of the original proposition).…

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1. Summarise the article in your own words. Which single claim would you verify first, and why?
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3. If two sources disagreed about one of these facts, what would you need in order to trust one over the other?

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