This is a fundamental and valid component of Postmodernism: There is no The Truth™ that we can know independently; once we pick an epistemic system, we have to simply define its results as, in some sense, small-tee "truth". A seeming corollary to this result is that all systems which its adherents call epistemic are equal. This corollary would be true, though, only if finding the truth were the only way to evaluate an epistemic system. But there are other criteria which can be employed, and which do differentiate purported epistemic systems into better and worse.
We can't directly evaluate an epistemic system on its ability to deliver The Truth™, but we can examine the results of an epistemic system to see if its output shares essential characteristics of our intuitive notions of truth. If its results are not even truth-like, we can evaluate the system as a worse epistemic system; if the results are truth-like, it's a better epistemic system.
There are two important components of our intuitive notions of truth: The truth should be the same for everyone, and the truth should be useful, at least in general.
The notion that everyone has her own truth is one of the more stupid notions from postmodernist philosophy that has slipped into the general population. We have a perfectly good word to represent what everyone has her own of: opinion. If you want to espouse epistemic nihilism, then just say there is no truth and everyone has only his or own opinion. The problem with saying that everyone has her own truth is that it equivocates the normative sense of truth, that the truth should be the same for everyone. It's not enough to claim to strip the normative sense from "truth"; opinion already lacks that normative sense: It's hard to see the use of a word with a normative connotation in place of a perfectly good word without that normative connotation as anything but intellectually dishonest.
There are three ways of approaching the normative connotation of "truth". The first is to simply arbitrarily privilege an epistemic system and coerce everyone into believing its results. There's nothing intrinsically wrong with coercing conformance to ethical norms—we put murderers in prison rather than engaging them in philosophical argumentation—but applied to epistemology this approach—as a millennium of effort by Christianity and Islam, as well as "Soviet science", to suppress "heresy" and schism attests—proven ineffective.
The second approach is to simply declare that whatever everyone agrees on is in some sense truthful by definition.
The third approach is to pick an epistemic system that generates principled agreement in a logically deterministic way: It is possible to determine the outcome of the epistemic system, and thus everyone who applies the epistemic system in this deterministic manner gets the same answer. If you get a different answer, it should be possible to identify where you are not using the epistemic system as specified.
The Scientific Method is a good example of an epistemic system which generates principled agreement. To recap, the key components of the public version of Scientific Method are:
- The Scientific Method accepts as foundational evidence only statements of perception about which everyone agrees (see ).
- A scientific theory is a collection of axioms which entail as theorems the foundational evidence (and thus entail as non-theorems the contrary of the foundational evidence).
- A theory with greater scope, i.e. that entails more statements of evidence, is better than a theory with lesser scope
- A simpler theory, i.e. a theory with fewer irreducible axioms, is better than a theory with more axioms
Since each and every step in the Scientific Method is deterministic and univocal, if two people disagree on the outcome of this epistemic system then we can confidently conclude that one of the people is not applying the method as specified.
Contrast this epistemic method with the theistic pseudo-epistemology of scripture and revelation. It's the case that, if you accept a scripture and some extremely elaborate and rococo interpretive system, you can generate principled agreement, but the obvious flaw in this method is the choice of scripture and interpretive schema (exegesis). There's simply no way to generate principled agreement on which scripture to accept: The Catholic Bible? The Protestant Bible? The Greek or Russian Orthodox Bible? The Book of Mormon? The writings of Mary Baker Eddy? The Koran (and which hadith)? The Upanishads? The Buddhist scriptures? And that's to name simply a few, and completely ignore the minor and more-or-less heretical "prophets" () and non-canonical scriptures. And once you've chosen a scripture, how do you interpret it? Few people take Dueteronomy 21:18-21 literally, but once you abandon the literal meaning, how do you decide in a principled, deterministic way which metaphor to employ?
These are... nontrivial... issues, as the history of religion has made blatantly obvious. The "metaphysical buy-in" to any religion is enormous, everyone seems to buy in at least a little bit differently, and there are many ways to buy in that are enormously different.
The Scientific Method requires its own metaphysical buy-in, of course. But there is only one scientific method to buy into, the buy-in is cheap, and, since the scientific method is nothing but formalized common sense, which ordinary people apply to their daily negotiation with reality, most everyone has already bought in at least 90%.
 We can see this sort of dishonest equivocation in action in the attitudes of woo-woo New Age dumbasses such as Deepak Chopra to skeptics. If their "everyone has her own truth" stance were consistent, if "truth" really were standing in 100% for "opinion", then the opinions of skeptics would be just as good as their own woo-woo, and they wouldn't complain about sjkeptics so vociferously.
I refer this sort of equivocation as the "Postmodernist two-step":
- All epistemic systems are equally valid, therefore my epistemic system is a priori valid
- My epistemic system contradicts your epistemic system
- Since by (1) my epistemic system is valid, yours must be invalid
 Or, more precisely, "This (red) apple appears to be a different color than that (green) apple.