Are these two logical negation of each other

first-order-logiclogicpropositional-calculus

Are these two statements considered negation of each other?:

  1. All fruits are sweet.
  2. Some fruits are not sweet.

I guess there is no relation between the two. I guess we can only say that statement 1 renders statement 2 "invalid" and vice versa. But nothing else can be said (in terms of "negation" and any other), am I right ?

Best Answer

If you formalise these two statements in first order logic, they read something like:

  1. $\forall f \cdot F(f) \to S(f)$

  2. $\exists f \cdot F(f) \land \lnot S(f)$

where $F(f)$ asserts that $f$ is a fruit and $S(f)$ asserts that $f$ is sweet. The logical negation of 1 is $\lnot\forall f \cdot F(f) \to S(f)$ which (in classical logic) is logically equivalent to 2. Likewise the logical negation of 2 is equivalent to 1.

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