An original editorial by Dawson Smith
In all the debate over the language of Trelawney's first prophecy (see the editorial Chosen, which covers the debate very well), one important aspect has been discussed very little, if at all. That is, how do the prophecies work? Well, first off, there are a couple of terms commonly used by Literature and Philosophy majors alike (at least at NYU). Namely, the three concepts for classifying predictions, assuming they have a basis in truth: self-defeating, self-fulfilling, and futile (or "Cassandra") prophecies.
Self-fulfilling prophecies are, by the strictest definition, predictions which come to pass by the very virtue of their being stated at all. A simple example would be, "I am going to finish this sentence." A related concept, more common in the real world, is that of self-propagating predictions, wherein a trend is already in place and the statement (e.g. "The stock market will keep plunging") aids the trend in its continuance.
Examples of self-defeating prophecies are easier to find. These are generally the doom predictions that, once stated, give people the chance to change the future. Warnings such as, "We're going to run out of fossil fuels in twenty years," or "Most Americans will never receive social security," while true in their basis, spur people toward finding a way to prevent them.
The third category is the futile prophecy, whereby the future is set and nobody can do anything to change it one way or another. This is also known as the "Cassandra" category, after the classic Greek character who was doomed to know the future but have no one believe her when she spoke it. While this is certainly not the case in the HP universe (they keep records, for one thing) the idea of predetermined futility seems evident in that the prophecies we've seen have come true, and there seems to be enough reverence towards them to believe that they always do. I have another theory.
To begin, we only have intimate knowledge of two "real" (by Dumbledore's words) prophecies, both Trelawney's. Now, aside from them both having been told in a trance-like state and both having to do with Voldemort, they would seem to have little in common. However, both of them were heard by listeners who attempted to prevent the foretold events from coming true, and in both cases the preventative measures in fact caused the forewarned happenings.
In PoA, Harry hears Trelawney's prediction of the Dark Lord's servant returning, and of course does not want this happen. This desire (admittedly along with concern for Ron's welfare) leads to the actions in the Shrieking Shack, which include "Scabbers" being unmasked, thereby forcing Pettigrew to return to Voldemort for safety. This allows Voldemort to soon become "greater and more terrible than ever he was."
In OotP, we learn of Trelawney's first prophecy, about the birth of the one with the power to vanquish the Dark Lord. We also learn of the circumstances surrounding the telling of it, and how Voldemort's spy listened in, and Voldemort attempted to kill Harry in order to circumvent the omen. We know the rest -- Voldemort thus marks Harry, "as his equal," and solidifies the prophecy he tries to prevent.
This could easily be coincidence, but we know that JKR doesn't traffic much in happenstance. Trelawney's trances, which we can assume are indicative of all prophecies, happen without her knowledge, and only even involve her by the most generous of definitions. We can easily assume from this that being a "true" seer is not so much a talent as a lucky fluke; the seers are merely conduits for something, which brings me to my final point.
Firenze attempts, to no real success, to explain the mysteries of the stars, and while he disposes of the normal concept of divination as "fortune-telling," he doesn't dismiss it entirely: "Trelawney may have seen. I do not know." This tells us a number of things. For one, we understand the centaurs' methods to be far more honed and accurate, though more vague, than the human methods; and yet, Firenze alludes to the experience of future-event-enlightenment, which can occur in certain humans. He doesn't scoff at this, he just raises tacit doubts that such an experience ever occurred to Trelawney.
That's right. Firenze, arguably the most knowledgeable entity on the subject, considers seeing to be an experience rather than an attribute, which in combination with his preference for astrology, might explain something. In literature, the phrase "when the stars align" has generally been used to mean "when all the elements are in place and the timing is just right." For my original theory to hold water, that self-fulfilling predictions are being disguised as self-defeating ones, all the elements would have to be in place, and the timing just right, for the prophecy to come true. Yet they must still hold the JKR theme of our choices and our actions deciding our fates. This is as far out on a limb as I will deign to go with all of this, but maybe the stars literally do have to be aligned in a certain way, and certain witches and wizards are blessed and cursed with reciting these celestially-ordained predictions at the right place and the right time in order to make sure that they occur. It's wild, and I don't expect you to accept all of this, but it does fit with all we know.