…the noun articulated by a “the” has a history: it comes again, and was foreseen; it doesn’t just occur, but re- and precurs. When “the bear comes out of the woods,” he’d been known or mentioned before; when “a bear comes out of the woods,” it’s somewhat more alarming, less expected—he has not appeared before, and the hearer starts a little (or a lot, depending on whether she is learning this fact in a field or in a reading chair). Articles thus operate as time signs: they cast their light ahead, onto their subsequent nouns, but cast a different light upon appearance (“a bear”) than upon reappearance (“the bear”). The “the” presumes something already there; it reacknowledges it. But an “a” makes its noun crop up on the spot: with an “a,” the unforeseen (and, by extension, the disappearing) is articulated.

….

If the “the” tells something about the recursive past (in which its noun existed before), and the “a” tells something about the precursive future (an unforeseenness about to befall), then a poet can administer such articles for their cursive and discursive powers, savoring the time relations they incur.

Heather McHugh “A Genuine Article”