Causality
A necessary cause is a cause that says: Whenever B occurs, A will also have occurred. Knowing that A occurred, on the other hand does not necessarily imply that B will also have occurred.  Example: Senators in the United States need to be at least 30 years old. People that are younger cannot become senators. Therefore, any senator is at least 30 years old. (On the other hand: there are people that are at least 30 years old  that are not senators).