Destructive Dilemma

A formal argument in Logic in which it is stated that

1. $P\Rightarrow Q$ and $R\Rightarrow S$ (where $\Rightarrow$ means ``Implies''), and

2. Either not-$Q$ or not-$S$ is true, from which two statements it follows that either not-$P$ or not-$R$ is true.

See also Constructive Dilemma, Dilemma