Truth changes. And our refusal to deal with that leads to a lot of epistemic dysfunctions.
"Franc, that's facking stupid", you're thinking. "How can truth change? If something is true, then it's true! Truth can't change!"
Actually, I suspect that this is a semantic issue more than anything else. I do not disagree that facts don't change. Facts are, after all, nothing more than a parcel of what we are studying. The fact that the Earth is round does not depend on any epistemic agent- it just is. But the understanding, the knowledge that the Earth is round does depend on the existence of epistemic agents. To likewise judge this constructed proposition as true requires other epistemic agents.
There is plenty of evidence available to support the idea that the Earth is round, even for pre-scientific civilizations. The movement of the constellations that differs depending on location, the horizon on the sea, the horizon when you stand on a high point, the shadow of the Earth during an eclipse, are four obvious ones. Eratosthenes was able to infer a rough number for the circumference of the Earth by using sticks and ingenious measurements at two very distant points. So this is not really rocket science.
And yet there must have been a first person to discover the fact, to go against the widely held religious belief (at least, before Antiquity) that the Earth was flat, which is most intuitive. There must have been an observation or series of observations, followed by an "aha!" moment. Then that person must have tried to convince other people, who may or may not have agreed with him.
This is perhaps not the best example, since the evidence is widely available. Take the discovery of Neptune. In the early 19th century, astronomical tables of Uranus were published, and it became obvious that the orbit of Uranus was perturbed in a way not explainable by the current model. Astronomers and mathematicians began to calculate where a planet would have to be in order to cause this perturbation. Examination of star charts revealed where Neptune was located.
Is the existence of Neptune a fact? Of course. Is it true? From our perspective, definitely. But for someone living in earlier times, where the only stargazing was done with the naked eye, any positive assertions about planets and stars would rightly be seen as spurious. Even though you know very well the facts of the matter, you would be hard-pressed to convince them of the existence of the nine planets, their orbits, their composition, their moons, and so on and so forth. I think we can go so far as to say that it would be irrational for them to take you at your word.
This of course brings up the issue- how do we determine truth? The only means we have, fundamentally, is that of rationality. We observe things around us, we integrate them with what we already know, and we come to new conclusions. Truth is therefore inextricably tied with our context of knowledge- what we already know and have integrated. The search for knowledge is not an ex nihilo creation but rather a networked expansion.
It must therefore be stressed that truth is relative. We must not, however, confuse "relative" with "truth relativism", which is the position that historical or cultural context determines truth. This is a collectivist concept, and only individuals discover facts. When we say that truth is relative, we simply mean that it is only reasonable for an individual to accept claims which are closest to his own context of knowledge.
Take the proposition "there are twenty planets around Alpha Centauri". It seems unlikely, but while we can't discount it out of hand, it would be foolish for me to believe it simply because I can state it. I can equally state that there are nineteen, fifteen, ten, five or zero planets. While obviously one of them MUST be true, I have no reason to accept ANY of these as true unless I have some evidence. Therefore I must refuse to accept a proposition which will eventually be considered true. This is not a problem, as long as one realizes that truth is relative.
This relativeness is very inconvenient for true belief. When you are trying to entrap people, you don't want them to judge things for themselves obased on their own context of knowledge, because most belief systems are unnatural and bear little relation to what we know. Instead, you want them to believe in a different kind of knowledge, a form of "revelation" for instance, which is wholly different from any knowledge you may have. There's your regular old facts and science (an altogether inferior form of knowing), and then there's something else you can't relate to or possibly argue with.
To posit such a dichotomy, the believers need to first eliminate the individual human element in this new epistemology. Christians just forget about either and set up their god as the source of knowledge- through "divine revelation", an inerrant Bible, or "divine intervention". Eastern mystics and New Age fans omit even consciousness, preferring to set up some abstract process as source of knowledge. Statists can't escape the human element but use collectivist concepts like "the people" and "the common good" to dissociate themselves from the context of knowledge. In either case, the source is said to transcend the evidence of the senses and make it irrelevant, which is self-contradictory since we still need the senses to interpret them.