Complex sentence: Mill also argued that free discussion is necessary to prevent the "deep slumber of a decided opinion" n would drive the onwards march of truth and by considering false views the basis of true views could be re-affirmed.
Split into simpler sentences: Mill also argued that free discussion is necessary to prevent the "deep slumber of a decided opinion". Discussion would drive the onwards march of truth and by considering false views the basis of true views could be re-affirmed.