is mortal" is true whether
is human or not; in fact, every proposition of this form is true. Thus the propositional function "if
is human,
is mortal" is "always true," or "true in all cases." Or, again, the statement "there are no unicorns" is the same as the statement "the propositional function '
is not a unicorn' is true in all cases." The assertions in the preceding chapter about propositions, e.g. "'