Why relativism is poison
The Sages talk about the importance of truth, and proper conduct in pursuing it. But they don't talk about relativism, which is a more modern doctrine. The idea of relativism is that what is true is relative to a person's culture, or group, or even to individual experience. There is no universal or objective "truth", but only truth for my group and for yours. This is sometimes presented as a healthy doctrine that promotes tolerance. Each person is entitled to his or her own view, and no one has a right to challenge him or her on it. And those who would challenge another are dogmatic "absolutists" about truth.
The problem is that relativism destroys the possibility of rational discussion and decision, including in trials, where witnesses are supposed to tell "the truth, the whole truth, and nothing but the truth," and where juries are supposed to determine the truth of guilt or innocence from the evidence and testimony. The common meaning of truth is that a statement corresponds to reality. I say "this is the year 2018" as I write, and it is. This doesn't deny fallibility but it does premise one reality: the world turns every day, and night follows day, whether we like it or not, whatever our opinion about it is.
Very often, those advocating relativism do so to immunize themselves from criticism. So recently Rudy Giuliani in an effort to defend President Trump from accusations of criminal behavior has said, "truth isn't truth", and "the truth is relative". But that this is a hypocritical ploy can be seen from the fact that he wants to be believed when he says that the accusers are "fake news". The reality is that when contradictions are accepted as not false, at desired by Giuliani, rational debate as a search for the truth becomes impossible. Anything goes, and power alone rules.