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How does the placement of a negation relative to a quantifier matter?

  • A.Negation may only be applied to atomic sentences
  • B.Negating a universal gives an existential of the negation
  • C.Negation and quantification commute freely in first order logic
  • D.Negating a quantifier removes it from the formula entirely

Answer: B

Not everything is F means something is not F, and not something is F means everything is not F, so pushing a negation through a quantifier exchanges the two quantifiers. That is the quantifier form of De Morgan's laws and it is what drives negations inward during clausal conversion. Commuting freely, disappearing and being restricted to atoms are all false. A rewriting rule that exchanges one operator for another, rather than merely moving it, is worth stating carefully because the exchange is easy to forget.

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