 Deontological theories tend to identify the right either with fixed laws or rules of conduct, such as the Ten Commandments, or with a single supreme principle of morality, such as the Categorical Imperative, understood as supplying a decision procedure in ethics. The attempt to specify substantive rules of right conduct for all cases founders on the need to make exceptions for different circumstances. “Thou shalt not kill” cannot be taken at face value, given the justifiability of killing in self-defense. Yet it is impossible to specify in advance all of the circumstances that could justify killing even in self-defense, given the complications that arise in, say, defensive warfare (e.g., problems of collateral damage). As social conditions change — for example, the technology and tactics of warfare, and our ability to affect the interests of distant others — rules of conduct that had been accepted in the past must be subject to revision, lest learning cease and people remain mired in dysfunctional habits (E 275–9). A method of moral inquiry is needed that can revise given rules, laws, and habits in light of new problems and circumstances. This method would take current and past customs and laws as starting points for moral theory, in conjunction with the history and anthropology of custom, the history of systematic theoretical reflection on morality, and the social sciences, which inform us of the nature of current problems, and the probable consequences of attempting to institute this or that new law or custom (E 178–9). Intelligent moral inquiry, while it begins with current customs and convictions about the right, treats them as hypotheses to be tested in experience.