The only way the concept 'the truth' can be plausibly denied is by rejecting the idea of an external reality. For sure, there are a number of ways of questioning the idea of a world outside of consciousness - but nobody actually believes an external reality doesn't exist for any kind of practical purpose. From the working assumption that this reality exists, it's a very simple leap in logic to say that 'the truth' exists and that individual perceptions partake of that truth to varying extents. It's about as clean-cut and direct as any epistemic argument can be - that these Platonic type forms of truth exist and that through our perceptions/interpretations we partake of that truth to varying extents. Your crazy aunt in a retirement home who thinks she's living on a cruise ship, probably partakes less than you.

wtfsvi: I'm not sure that Kant did prove that, but his failing to prove it gets at the root of all this. His positions on a priori knowledge are internally consistent, but unverifiable because he's made the assumption that these forms of knowledge exist. Which is the same thing, we have to make some sort of assumption to exercise logic, and if that assumption is going to be that a world external to us exists - that seems like a fairly sensible and intuitive assumption to me. I don't believe the universe being relativistic makes much difference here either, it's still external.

The only way, IMO, that the concept of meaningful truth can be challenged is to regurgitate the whole Cartesian doubt process as it applies to epistemology. Which is probably bottomless dealing as it does with axioms, but more to the point, is useless since the same rules would apply within that self contained frame even if we somehow discovered that all is consciousness.