A sufficient condition is one for which the following is true:
- If A is a sufficient condition of B, then if A is true, B must be true.
For example, "I heard thunder" is a sufficient condition of "There was lightning nearby," because if I heard thunder, there must have been lightning nearby to cause it. However, "There was lightning nearby" is not a sufficient cause of "I heard thunder," because there may have been lightning nearby that I did not hear.
From the affirmation of a sufficient condition, one may infer that the necessary condition is true.