QUOTE(Text @ Thu 3rd March 2011, 1:13am)
What does this mean?
If A has a value different from nothing then event B occurs. Then the opposite of event B occurs?
Since A only has to exist, and it always exists as a value, then B and its opposite always occur?
It means that if we can prove that:
* if A is true then B must be true
* B is false
then logically A must be false, yet some people persist in saying that A must be true.
For example, if John Smith lives in Des Moines, it follows that he lives in the USA. If we can prove that he doesn't live in the USA, he can't live in Des Moines. Yet there may be people who insist he does, maybe invoking WP:NOR.