In most expert systems, efforts are made to keep the rules in the knowledge base free from contradictions., because a logical system that generates even a single contradiction may collapse. We argue that not only can contradictions be tolerated, but in fact they are useful. An excellent test of an argument is to compare it with the best argument for the opposing view. Accordingly, we propose a multilevel architecture, where the object level may include contradictory rules, and the metalevels resolve these conflicts. Once such an architecture exists, there are advantages to allowing contradictions in wider contexts. In a legal context we may want to examine both sides of an argument. In administrative applications, we may need systems that look over a clerk's shoulder to check that he is following one of several plausible, but not necessarily compatible, approaches. We are currently implementing these ideas.