Here's a fragment from Jack London's Star Rover:
Wordsworth knew. He was neither seer nor prophet, but just ordinary man like you or any man. What he knew you know, any man knows. But he most aptly stated it in his passage that begins "Not in utter nakedness, not in entire forgetfulness.
To the best of my — admittedly sketchy — knowledge of grammar, the second sentence should read neither a seer nor (a) prophet, but just an ordinary man. Even if we accept the seer and the prophet as "poetic entities" ("father and son," "robber and robbed," "man to man," or whatever), logic would still dictate that ordinary man should be preceded by an an.
It is a well-known fact that Jack London's usage wasn't always up to par; his grammar, on the other hand, was always top-notch.
Logically, the sentence is grammatically incorrect (correct me if I'm wrong: no pun intended).
Illogically, it reads smoothly: intuitively, a native speaker understands that there's nothing wrong here.
However, those to whom English is not the first language cannot be expected to be intuitive in this case and would want an explanation. Any suggestions?