The Preposterous Trope

Dante conveys the speed of his ascent to the first sphere (Moon) (Paradiso 2) by saying it took no longer than it takes an arrow "to strike, fly, and leave the bow" (2.23-6). The reverse chronological order of these actions is an example of hysteron proteron, from the Greek for "later / first."

Cantos 21 and 22 of Purgatorio put Hysteron Proteron in play in various ways. It could be seen as the structuring principle of these cantos. Here's more about the trope:

The "classic" example is from Virgil's Aeneid, where the poet says, "Moriamur, et in media arma ruamus," or "Let us die, and rush into the midst of the fray (2.353)." Shakespeare is a master in the use of hysteron proteron. "Th' Antoniad, the Egyptian admiral,/ With all their sixty, fly and turn the rudder (Antony & Cleopatra 3.10.1-2)." In both instances the verbs are "out of order." You don't die before you go into the fight. You don't "fly" before turning the rudder. But the more vivid or prominent verbs are "die" and "fly."

A biblical example of this is in the highly-charged scene at the Last Supper when Jesus takes the bread and says to his disciples, "Take, eat; this is my body." In fact, the more "normal" way of saying it would be "This is my body. Take it and eat it." Hysteron proteron is a device that allows great immediacy of an important concept.

Another take on it:

Hysteron Proteron is the rhetorical equivalent of the theological truth taught by Jesus that the last shall be first and the first, last. In rhetoric it is a (literally impossible) situation where the idea suggested by the first word (usually a verb) must happen temporally after (hysteron means "latter") the idea suggested by the second word (proteron means "former" or "first"). Thus, the latter word will become the former word.

It is the "cart before the horse," which the Greeks call "Histeron proteron" and we call "Preposterous." Preposterous? Ah, it began to make sense. The word preposterous is made up of two Latin words, where the "posterus" (the "latter," equivalent to hysteron) is made "pre" (the "former," equivalent to proteron).