Why isn't Modus Ponens valid here I have the following:
$(\neg A \lor B) \rightarrow (\neg A \lor B) \\ (\neg A \lor B) \\ \vdash \neg A \lor B $
And in my mind this seems like a legitimate use of the Modus Ponens rule. But the textbook I'm using disagrees.
Why is this wrong?
Edit:
The textbook has the following line in it as a part of a larger exercise:

The exercise asks to state whether the use of a giving rule is valid in each case. In the answer section it is marked as an invalid use.
 A: Note that $$(\lnot A \lor B) \rightarrow (\lnot A \lor B) \equiv \lnot(\lnot A \lor B) \lor (\lnot A \lor B) \equiv \top$$ In other words, the first premise is a tautology. It says nothing more than "either $\lnot(\lnot A \lor B)$ or else $(\lnot A \lor B)$ holds.  And in assuming the law of the excluded middle, one of the two disjuncts must be true, and thus the entire  statement is tautologically true.
The second premise is $\lnot A \lor B$.
The argument then can be stated as follows:
$\quad \top\tag{premise 1} $
$\quad \lnot A \lor B\tag{premise 2}$
$\therefore \lnot A \lor B\tag{repetition of premise 2}$
So the conclusion becomes a reiteration of the second premise.
Perhaps that is what your book was attempting to convey?
A: To infer $p$ from $p \rightarrow p$ and $p$ is a legitimate application of modus ponens. It isn't a particularly useful inference, but it is a correct one.
A: It doesn't appear to be wrong. If $C=C'=\neg A\vee B$, then a correct use of modus ponens is simply $(C'\to C)\wedge C'$ gives $C$, which is what you have.
