Servius, the 4th century commentator on Vergil, noted that "the art of poetry is not to say everything" (ars poetica non omnia dicere, on Aeneid 1.683).

This piece and your response, Matthew, suggests that it may be even truer to say that the art of poetry is not to know everything. Seems like a good epigram: ars poetica non omnia scire.