 where \(x_{1}\), …, \(x_{n}\) are all the variables free in either \(\phi\) or \(\psi\). And let \(\phi(t_{1},\ldots,t_{n})\) and \(\psi(t_{1},\ldots,t_{n})\) result by the simultaneous substitution of terms \(t_{1}\), …, \(t_{n}\) for \(x_{1}\), …, \(x_{n}\) in, respectively, \(\phi(x_{1},\ldots, x_{n})\) and \(\psi(x_{1},\ldots, x_{n})\); changing bound variables as necessary. Then the rules of inference governing \(\mathcal{D}\) are simply these: