Assume that x happening is necessary. You can never conclude anything simply by knowing that x happened. However, you can draw a valid conclusion if you know that x did not happen.
Now assume that x happening is sufficient. You can never conclude anything simply by knowing that x did not happen. However you can draw a valid conclusion if you know that x did happen.
Example 1: To reduce blood pressure, Pam will need to moderate her salt intake. (necessary condition)
If we know that Pam moderated her salt intake, we know nothing about whether Pam reduced her blood pressure. Because it's possible that there are conditions necessary beyond that one necessary condition.
But if we know that Pam did not moderate her salt intake, it's time for us to conclude that Pam did not reduce her blood pressure. The necessary condition has not been satisfied.
Example 2: If Pam moderates her salt intake, then Pam will reduce her blood pressure. (sufficient condition)
If we know that Pam did not moderate her salt intake, we know nothing about whether Pam has reduced her blood pressure. Because it's possible that Pam can reduce her blood pressure without moderating her salt intake.
But if we know that Pam moderated her salt intake, then that's all we need to know. Pam reduced her blood pressure since the sufficient condition has been satisfied.
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