is always true. Thus the statement "all men are mortals" is the negation of the statement that the function "
is an immortal man" is sometimes true. And the statement "there are unicorns" is the negation of the statement that the function "
is not a unicorn" is always true.[38] We say that
is "never true" or "always false" if not-
is always true. We can, if we choose, take one of the pair "always," "sometimes" as a primitive idea, and define the other by means of the one and negation. Thus if we choose "sometimes" as our primitive idea, we can define: "'