A rule-based program will return a set of answers to each query. An impure program, which includes the Prolog ut !'' and not@รA'' operators, can return dierent answers if its rules are re-ordered. There are also many reasoning systems that return only the ยฎrst answer found for each query; these ยฎrst answers, too, depend on the rule order, even in pure rule-based systems. A theory revision algorithm, seeking a revised rule-base whose expected accuracy, over the distribution of queries, is optimal, should therefore consider modifying the order of the rules. This paper ยฎrst shows that a polynomial number of training labeled queries'' (each a query paired with its correct answer) provides the distribution information necessary to identify the optimal ordering. It then proves, however, that the task of determining which ordering is optimal, once given this distributional information, is intractable even in trivial situations; e.g., even if each query is an atomic literal, we are seeking only a perfect'' theory, and the rule base is propositional. We also prove that this task is not even approximable: Unless P NP, no polynomial time algorithm can produce an ordering of an n-rule theory whose accuracy is within n c of optimal, for some c b 0. We next prove similar hardness and non-approximatability, results for the related tasks of determining, in these impure contexts, (1) the optimal ordering of the antecedents; (2) the optimal set of new rules to add and (3) the optimal set of existing rules to delete.