Chapter Eight, Tutorial Four
PD+ includes all the rules of SD+ and all the rules of PD. In addition, there is one new rule of replacement: QN. (In case you've forgotten, click here for a pop-up of the older rules of replacement introduced in chapter 5)
Quantifier Negation: the rule QN
You have already noticed that to say something like "not everyone is present" ('~(^x)Px') expresses the same idea as "someone is not present" ('(%x)~Px'). These two sentences of PL are logically equivalent: we have shown (8.2ex V) that from either one, the other can be derived.
In general, moving the tilde from one side of a quantifier to the other and changing the type of quantifier (from universal to existential or vice versa) always provides a logically equivalent sentence. The rule, "quantifier negation" or "QN", codifies this equivalence:
Any accessible line which includes a tilde and a quantifier immediately following the other is a candidate for QN.
Now, consider the derivation beginning with one premise:
From the justification 1 QN, which of the following can be derived on line 2?