I want to know the definition of null quantification and when and why we need to use it. In a book I was learning, it says:
"establish rules for null quantification that we can use when a quantified variable does not appear in part of a statement. "
And
" Establish these logical equivalences, where x does not occur as a free variable in A. Assume that the domain is nonempty.
a) $\forall x ~( P(x) \lor A ) \iff ( \forall x~P(x) ) \lor A$ "
Now can anyone explain me what is said in this two quotes. And what's the $A$ here in the second quote. Is this stands for a proposition without $x$ or a free variable or a bound variable.
Please explain this with a proper definition and a clear example.
$\endgroup$ 42 Answers
$\begingroup$In first-order logic (FOL) a well-formed formula (wff) is also called a sentence or a proposition (terminology originated mainly from Aristotle and Greek stoics) if and only if it has no free variable(s) occurrence since all its quantified in-scoped variables are all bounded in their corresponding domain of discourse (using philosophy jargon we say a true proposition must be grounded or corresponded to ontological fact manifested in extensional objects). A null quantification is related to such propositional (zeroth-order) case while a general wff with free variable occurrences is not truth-functional, it can only be satisfied or unsatisfied by some particular value(s) of its bounded variable(s).
Just bear in mind below first-order equivalences (with one exception if noted). So we can mentally imagine the null quantification case as a limiting case of fully bounded case and we can have first-order equivalence (4) in this limit case while we cannot have such nice first-order relationship in the general wff case (2).
(1) $\forall x ~(P(x) \land A(x)) ≡ \forall x~P(x) \land \forall x~A(x)$
(2) $\forall x ~(P(x) \lor A(x)) \not≡ \forall x~P(x) \lor \forall x~A(x)$
(3) $\forall x ~(P(x) \land A) ≡ \forall x~P(x) \land A$
(4) $\forall x ~(P(x) \lor A) ≡ \forall x~P(x) \lor A$
$\endgroup$ $\begingroup$$A$ is a proposition (a well formed formula) and it explicitly states it is being used "where $x$ does not occur as a free variable in $A$."
So we have a part of the statement, $P(x)\vee A$, where the term $x$ does not occur as a free variable; that part being the $A$.
We would like to establish a rule if inference where we can 'extract' this part from the quantified statement -- also, conversely, to 'inject' it when we have the disjunction.
That is we wish to show that $\forall x~(P(x)\lor A)$ entails, and is entailed by, $(\forall x~P(x))\lor A$. That the two statements are equivalent (well, when the domain is non-empty).
$\forall x~(P(x)\vee A)$ claims that: "Every valuation for $x$, in this non-empty domain, satisfies the formula $P(x)\lor A$."
$(\forall x~P(x))\vee A$ claims that: "$A$ is satisfied or every valuation for $x$, in this non-empty domain, satisfies the formula $P(x)$."
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