Epistemological relativists will refer to "different ways of knowing" and claim the equal validity of the knowledge obtained from radically different epistemologies. I would like to point out that there is a distinction between truth and opinion. People are entitled to their own opinions. But in our discourse about reality, our claims are either true or false. NOTE: there is no true only from my standpoint or true only from your standpoint--we call this sort of thing "opinion" not truth. I may think chocolate is a superior flavor of ice cream, but this is a matter of opinion. I would not submit it as a fundamental truth. I would not expect others to agree with my opinion. But if I say that the earth is made of marshmallow, this is a statement about reality and the truth value of the statement can be determined to be false. Or if I say that jumping off a 100 story building without a parachute or other protection to the concrete below will not result in the least injury because gravity is not real, again, this is a statement about reality and the truth value of the statement can be shown to be false. The earth was once thought to be flat, but overwhelming evidence has shown that ships don't sail off the edge into the void--the earth is round. Truth is not relative. You can believe that tigers are harmless herbivores, but the truth value of your belief will become apparent if you encounter a hungry tiger in the wild.
Postmodernist Relativism draws upon universal skepticism, which even Russell conceded can not be refuted, but then, Russell saw universal or radical skepticism as a "barren" philosophy. Of course nothing can be known with 100% certainty. We could all be the victims of Descartes' evil genie, but it's irrelevant--how would we ever know? Should we be paralyzed by uncertainty or abandon any notions of true and false because 100% certainty can never be attained? What we see is a representation of reality, as constructed from sensory data and interpreted by the brain. The brain's model of the world is an analogue to reality, and it is a sufficiently accurate approximation for us to know that some things work and some things don't. Postmodernists like Feyerabend have said that science is just another form of social discourse, and that since scientific theories are in principle subject to revision, "anything goes." What facile nonsense. Even a child understands that though there may be more than one way to swim, there are definitely wrong ways to swim, and the difference can mean life and death. From the viewpoint of radical skepticism and its permutations, relativism and postmodernism, the entire epistemological enterprise is meaningless. If all methodologies are simply forms of social discourse and if all truths are equally true, then the term "truth" ceases to have any useful meaning. Truth is however not whatever we wish it to be. Carl Sagan goes over this particular fallacy in some depth in "The Demon-haunted World." I would recommend his book as a treatise on critical thinking.