To believe a person is not necessarily to treat him as an original authority. He is an original authority on what he himself has done and seen and heard: I say an original authority because I only mean that he does himself contribute something, e.g. is in some sort a witness, as opposed to one who only transmits information received. ... Thus a speaker may be a total original authority for the fact that he gives, as would usually be the case if one of us said he had eaten an apple, or an original authority, but not a total one, as if he says he saw some of Leonardo’s drawings; or he may not be an original authority at all, as if he says that Leonardo made drawings for a flying machine.
Is there a special usage of "an" at work here? Does it mean something like "not fully"?