Not truth, not knowledge, not even the physical world that surrounds us. Everything is relative to the senses, perceiving the world around them, and relative to the mind, creating a moralistic and logical base that it processes information through. Ideas and knowledge formed in the minds of men are subjective depending on the mind of the man they originate from, and the interpretations made by the minds on the receiving end.
The physical world is often taken for granted as being completely and absolutely true: there are not multiple versions of the world perceived between different people and it is a solid object, and different objects cannot occupy the same space at the same time. However, even the physical world, the least nebulous realm, open to the least interpretation, is not absolute. On a quantum level the universe is quite different. Space and time are a foamy irregular mass, particles flash in and out of existence, and atoms can spin up and down at the same time. Quantum wave forms prove that a card cannot be balanced on its edge, even in a perfect environment, and that it will fall to both sides at the same time. The best example of an absolute truth, the physical world, is even relative.
Our senses are also relative, and rely on many variables that can skew peoples" perception of reality. The sense that people typically rely on most, sight, is nothing more than light emitting from a source and then reflecting off of another at just the right trajectory as to be funneled into the back of the eye. From there, it stimulates a receptor, and a signal of electricity and chemical reactions enters the brain, which has a maelstrom of its own electrochemical reactions to decipher the incoming data. These millions of variables make perception completely relative, even when another person could be in the exact place at the exact same time and perceive something completely different because just a single variable may have been off, or unique to the perceiver.