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Those who practice relativism do not considerably seek to improve or correct their epistemic judgements,
...and so rational reflec­tion and dis­course become not only pointless for them, but also impossible for them. 

In every instance the possibility of rational thought or advancement towards actual comprehension, 
….necessarily requires that there are grounds for distinguishing good or bad, relevance or irrelevance, truth or lies. 

By denying that requirement,  relativism also precludes reason itself.