A few days ago, I posted a note from a friend of mine about openings which included mention that she's not a fan of prologues. Several of you commented on this, and this was a hot topic in our recent structure workshop, so I thought I would elaborate a little on why prologues can be problems.
Stories unfold in time. This might seem like a Duh statement -- of course, stories unfold in time -- but given the state of so many unpublished manuscripts, this is perhaps not as obvious as we might hope. We see evidence over and over again of authors who don't contemplate the time factor, much less control it in a way that makes sense for the story.
Aristotle (I mention this name, and a collective groan arises from the structure workshoppers) said that stories must be unified in three ways: time, place, and action. What this means has been the subject of some scholarly debate in the millenia since the Poetics first came into being. For example, initially the unity of time seemed to encompass only histories, stories in which true historical events occurred in the same set time span, whether or not those events were causally linked. Later scholars read Aristotle's notes about unity of time in conjunction with his statements about causation and beginnings, middles, and ends, and concluded that the unity of time extended beyond the boundaries of historical narration into fiction.
At its most extreme, during the Renaissance the principal of the unity of time was held to require all story events to take place over the course of a single day. Even then, most storytellers violated the single-day rule. Nevertheless, the underlying principal was recognized as valid: stories unfold in time, and compressing that time can enhance the story.
So, if you imagine your story as a timeline, much like those we used to see in our high school history textbooks, you can plot the story events along the timeline: Day one, Franz Ferdinand is assassinated and Austria declares war on Serbia; Day Three: Russian declares war on Serbia; etc.
Now, here's the problem with prologues. In most prologues, there's generally a big time gap between the event in the prologue and the event in chapter one. It would be as if Franz Ferdinand had been assassinated in 1914, but nobody declared war over it until 1922. The timeline gaps, the causation starts to fade, and the connection between the beginning and the middle of the story becomes less obvious.
This doesn't mean that all prologues are evil and must be destroyed for the good of all humanity. What it does mean is that any time you feel the urge to write a prologue, you must test whether it would be better to eliminate that gap in time. Or in place. Or in action. Sometimes just moving the events forward in time and calling them chapter one is the solution. Sometimes cutting the prologue and treating those events as back story is the solution. And sometimes, the prologue really is the best way to go -- but honestly, there are far fewer legitimate prologues than sagging timelines in most slush piles.