The historicist's useful fiction

Bill Benzon's post on aesthetic vs. ethnographic criticism, which notes that the ethnographic critic "simply needs to be interested in culture wherever and however it is," leads me to wonder about one of the ethnographic critic's key but frequently unstated difficulties: not what any given author knew, but what s/he likely did not know.  The historicist critic, in particular, is easily tempted to speak of what an author "probably" knew, or "certainly" knew, or maybe "must have" known.  This historical novelist must have been aware of that ongoing theological controversy; that poet surely was acquainted with this other contemporary political debate; etcetera, etcetera, etcetera.  It's very tempting, in other words, to posit a completely informed author--especially since imagining such an author justifies us in situating his work in a purportedly relevant "context."  We don't really have a critical language for dealing with authors who might have been (at best) only partly aware of their current intellectual and political surroundings, or even (at worst) completely oblivious to them.  And yet, even a quick moment of introspection will reveal the massive gaps in our own understanding of something as undemanding as, say, contemporary pop culture, let alone current politics or even the scholarship outside our own field.  Some resemblances among discourses are accidents, after all, not signs of contact between high and low culture or political and literary languages.