There is a difference though between saying that the intentions the author evinces in writing his or her text are the meaning of the text, and saying that the text reveals a good deal about the intentional values held by the society that the text represents. Language is public and we do not control it, despite what Humpty Dumpty says. The author does not control the meaning of what is written, but that certainly does not mean we cannot investigate those meanings.
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Literary theory often urges us to ignore what the author believes or says about his or her own work, and to look at the text itself. Yet many literary theorists (perhaps not necessarily good ones, but many nonetheless) couch their analyses in terms of agency - like commenting of a horror movie that "The fact that the black man dies first tells us that black people do not have a place in this society", or, to quote an example our professor gave us, "The buttons in a tram that signal for the driver to stop are intended to train you into behaving in an impersonal and instrumental way towards servants." If we are supposed to ignore what the author of these artifacts says about them (which is almost certainly at odds with the "analysis"), how can we then coherently speak about intention, suggestion, or other notions of agency? Who is telling or intending, if not the creator of an artefact?