There should be something wrong with the definition in the textbook. Must be a conditional expression with "P→Q".

Here you can easily think that P and Q are two propositional variables. In fact, both P and Q here are propositional formulas, and it is more appropriate to write A → B. That is to say, the former and the latter are related, such as P∧Q→P, which is a tautology.

When both P and Q are propositional arguments, of course, P→Q can't be tautology.