 Before we look at a wider range of examples, a detailed example involving quantified variables will prove to be instructive. Whitehead and Russell follow Peano’s practice of expressing universally quantified conditionals (such as “All \(\phi\)s are \(\psi\)s”) with the bound variable subscripted under the conditional sign. Similarly with universally quantified biconditionals (“All and only \(\phi\)s are \(\psi\)s”). That is, the expressions “\(\phi x \supset_x \psi x\)” and “\(\phi x \equiv_x \psi x\)” are defined as follows: