In this essay Rawls introduces a distinction between justifying a practice and justifying particular actions within a practice. The distinction is important, for it shows how utilitarians might rebut the common criticism that they are committed to endorsing acts like punishing the innocent and breaking promises when doing so will maximize the good. The main idea is that utilitarianism offers a justification of practices, and particular actions are justified not by appealing to utility but rather by appealing to the rules of the practice. So there is a utilitarian justification for having a ‘punishment practice’ in which only the guilty are punished, and, indeed, for having a punishment practice in which utilitarian considerations do not enter into decisions about who should be punished. Similarly, there is a utilitarian justification for having a ‘promising practice’ in which the obligation to keep one’s promises is not dependent on considerations of the utility of keeping one’s promises. Utilitarianism has seemed so implausible to many because they have mistakenly construed it as a justification of actions and not practices.
At first glance Rawls’s position might seem like a version of rule-utilitarianism, but it is importantly different from this view and, I think, intuitively more plausible. The rule-utilitarian suggests we should perform those actions which, as a rule, maximize utility, and as a rule (in general) punishing the guilty and keeping one’s promises maximize utility. This view has always been open to the charge of irrelevance–why should we perform an action we know is sub-optimal just because it is usually better to perform that action than not? Rawls’s way of defending utilitarianism avoids this objection, albeit by making utilitarianism a less comprehensive engine of justification.