Oh, Mr. Werewolf, how you disappoint me.
Imagine that you get a job as a semaphore operator: one of those people who runs around on the runway signaling airplanes with batons. One day, you send the signal for "turn right" when you mean "turn left". The airplane turns right, and as a result, crashes into the terminal. Dozens of people are killed and hundreds injured.
The port authorities conduct an inquiry. Multiple witnesses, from the pilot to the airport control tower to semaphore operator on the next runway over, testify that you signaled the semaphore for "turn right". There is even video evidence that you signaled "turn right."
"But --- but --- I'm the author of the signal! And what I meant is to turn left!"
"Holy shit," says the interrogator, "how stupid are you? It doesn't matter what you meant. What matters is what you signaled. Your arms signaled turn right; the sticks signaled turn right; the visual image telegraphed by your action was to turn right."
You are summarily fired.
Language is a communicative medium --- a system of signals. The meaning of an utterance is not determined by the intent of the author, but by the meaning that the interpretive community applies to the relevant system of signals. Ink on paper has an objective existence outside of the author's head, just as a semaphore signal does, and the patterns of ink on paper acquire meaning in the context of an interpretive community (viz., English speakers, or whatever) independently of any vaporous and transient firing of neurons in their originator's head.
This is not a postmodernist idea. It is not even a modernist idea. It is trivial common sense. To claim otherwise is to support Humpty Dumpty's contention, in Alice in Wonderland, that "When I use a word, it means just what I choose it to mean --- neither more nor less." If you believe in the intentionalist fallacy, you deserve to be one of the people who dies when the airplane crashes into the terminal.
Tune in next time for the explanation for why so many otherwise intelligent and reasonable people think that, for example, a novelist's intention towards his or her work has any special authority.