Logic / Unit 6 Logic 265 short lesson, about 20 seconds
Quantifier Negation
Captions available
The idea
~(x)Fx :: (∃x)~Fx, and ~(∃x)Fx :: (x)~Fx.
In practice
"Not everything is F" means something isn't. "Nothing is F" means everything isn't.
Where people slip
Pushing ~ through a quantifier flips it. De Morgan's rules (204), one level down — same flip, same forgetting-hazard.
Your place on the trail
Every lesson rests on earlier ones and opens later ones. Tap any to jump.
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