 is invalid in free logic; for even if every object in \(\bD\) satisfies \(A\), if \(t\) does not denote a member of \(\bD\), then \(A(t/x)\) may be false. (Here and elsewhere \(A(t/x)\) is the result of replacing all occurrences of \(x\) in \(A\) by individual constant \(t\); if there are no such occurrences, then \(A(t/x)\) is just \(A\).) Likewise invalid is existential generalization: the principle that from a formula \(A\) containing a singular term \(t\) we may infer that there is something in \(\bD\) that satisfies \(A(x//t)\), the result of replacing one or more occurrences of \(t\) in \(A\) by \(x\):