<Dd> For the whole numbers greater than two, being odd is necessary to being prime, since two is the only whole number that is both even and prime . </Dd> <Dl> <Dt> Example 3 </Dt> <Dd> Consider thunder, the sound caused by lightning . We say that thunder is necessary for lightning, since lightning never occurs without thunder . Whenever there's lightning, there's thunder . The thunder does not cause the lightning (since lightning causes thunder), but because lightning always comes with thunder, we say that thunder is necessary for lightning . (That is, in its formal sense, necessity doesn't imply causality .) </Dd> </Dl> <Dd> Consider thunder, the sound caused by lightning . We say that thunder is necessary for lightning, since lightning never occurs without thunder . Whenever there's lightning, there's thunder . The thunder does not cause the lightning (since lightning causes thunder), but because lightning always comes with thunder, we say that thunder is necessary for lightning . (That is, in its formal sense, necessity doesn't imply causality .) </Dd> <Dl> <Dt> Example 4 </Dt> <Dd> Being at least 30 years old is necessary for serving in the U.S. Senate . If you are under 30 years old, then it is impossible for you to be a senator . That is, if you are a senator, it follows that you are at least 30 years old . </Dd> </Dl>

The condition represented by the antecedent of a conditional statement