Definition df-nf

Description: Define the not-free predicate for wffs. This is read " x is not free in ph ". Not-free means that the value of x cannot affect the value of ph , e.g., any occurrence of x in ph is effectively bound by a "for all" or something that expands to one (such as "there exists"). In particular, substitution for a variable not free in a wff does not affect its value ( sbf ). An example of where this is used is stdpc5 . See nf5 for an alternate definition which involves nested quantifiers on the same variable.

Not-free is a commonly used constraint, so it is useful to have a notation for it. Surprisingly, there is no common formal notation for it, so here we devise one. Our definition lets us work with the not-free notion within the logic itself rather than as a metalogical side condition.

To be precise, our definition really means "effectively not free", because it is slightly less restrictive than the usual textbook definition for not-free (which only considers syntactic freedom). For example, x is effectively not free in the formula x = x (see nfequid ), even though x would be considered free in the usual textbook definition, because the value of x in the formula x = x cannot affect the truth of that formula (and thus substitutions will not change the result).

This definition of not-free tightly ties to the quantifier A. x . At this state (no axioms restricting quantifiers yet) 'non-free' appears quite arbitrary. Its intended semantics expresses single-valuedness (constness) across a parameter, but is only evolved as much as later axioms assign properties to quantifiers. It seems the definition here is best suited in situations, where axioms are only partially in effect. In particular, this definition more easily carries over to other logic models with weaker axiomization.

The reverse implication of the definiens (the right hand side of the biconditional) always holds, see 19.2 .

This predicate only applies to wffs. See df-nfc for a not-free predicate for class variables. (Contributed by Mario Carneiro, 24-Sep-2016) Convert to definition. (Revised by BJ, 6-May-2019)

	Ref	Expression
Assertion	df-nf	$$⊢Ⅎx\phi \leftrightarrow \left(\exists x\phi \to \forall x\phi \right)$$

Detailed syntax breakdown

Step	Hyp	Ref	Expression
0		vx	$$setvarx$$
1		wph	$$wff\phi$$
2	1 0	wnf	$$wffℲx\phi$$
3	1 0	wex	$$wff\exists x\phi$$
4	1 0	wal	$$wff\forall x\phi$$
5	3 4	wi	$$wff\left(\exists x\phi \to \forall x\phi \right)$$
6	2 5	wb	$$wff\left(Ⅎx\phi \leftrightarrow \left(\exists x\phi \to \forall x\phi \right)\right)$$