Necessary condition

From Conservapedia

A necessary condition is a condition in logic for which the following is true.

If A is a necessary condition of B, then if B is true, A must be true.

Thus, "Joe is alive" is a necessary condition of "Joe is talking," because if Joe is talking, then Joe must be alive.

However, "Joe is talking" is not a necessary condition of "Joe is alive," because if Joe is alive, he may or may not be talking.