To draw the line between what is considered a principle and what is considered a rule, as Dworkin does, is to show the difference between ideals and facts, or at least logically backed thought. Principles are very nice oughts, they possess a kind of dream land quality that creeps towards moral debate, and starts grappling with right and wrong. Rules on the other hand tend to be more cut and dry, less arbitrary, and also keep more distance from moral questions. That is not to say that rules do not attempt to control behavior or that they favor certain actions over others, but that rules are more concrete and a great deal of the time are geared towards action. Rules have a plan; they offer a systematic approach to accomplishing the goal that may not be necessarily laid out within them, but they are fairly independently conclusive. Principles on the other hand have to be weighed. Some things are considered better objectives than others. This ordering of what is a more just or a more beneficial outcome can lead to inconsistency within the application of rules, but on the other side of that, always enforcing rules no matter the consequences hardly seems the best approach. Rules do not leave a lot of wiggle room. That being said about rules, I do not think that law should be without principle.