The other reason people deny there is an objective truth is because they are skeptical that we know the truth. I accept that we never know what the truth is for sure. We can be mistaken. But to be mistaken, there has to be an objective truth! The idea of a mistake is that there is a truth and we have it wrong. Denying there are true ideas also denies that there are false ideas and mistakes. As a consequence, it prevents us from finding and correcting our errors, because errors are deviations from the truth and they say there is no truth to deviate from. Because they prevent error correction, relativism and other denials of truth are irrational ways of thinking. Similarly, denying there is a truth involves denying we can make progress, because progress means getting closer to the truth and understanding more of the truth.