 A formula \(A\) is closed if, and only if, all of its variables are bound. Otherwise, \(A\) is open. A variable \(y\) is free for \(x\) if, and only if, no free occurrences of \(x\) lies within the scope of a quantification on \(y\), \(\forall y\). We write \(A(y/x)\) for the formula that results from \(A\) when every free occurrence of \(x\) in \(A\) is replaced by an occurrence of the variable \(y\). A sentence is a closed formula of the language.