Witness escape
= Witness escape
An invalid quantifier inference that exports a formula still referring freely to a temporary existential witness. From $\exists x\,S(x)$ one cannot infer $S(v)$ for an independently named object $v$. A two-element interpretation with exactly one $S$-object refutes that inference. Introducing another quantifier after the invalid step does not repair it; the introduction must take place before the witness scope is closed.