As we noted in Section 1.1.4, the significance of statements being logically equivalent
is that they are different ways of saying precisely the same thing. If p is logically equivalent
to q, then knowing p is true guarantees that q is true, and vice versa.
If p ↔ q is a tautology, what does that say about p → q and q → p separately?5 If
p ↔ q is a tautology, then p → q and q → p must both be tautologies, too. Loosely
speaking, the truth of p is sufficiently strong to imply the truth of q and vice versa. That
is, in any case where p is true, it can also be noted that q will, without exception, be true,
and vice versa.