This sort of gets at the difference between act utilitarianism and rule utilitarianism. Act utilitarianism requires moral agents to dwell on the consequences of each and every moral action. Rule utilitarianism, by contrast, requires that utilitarian reasoning be applied to the construction of moral rules rather than the evaluation of moral acts. Basically, rule utilitarianism calls for the creation of 'moral rules-of-thumb' that, when followed, lead to the greatest amount of happiness. Obviously, following moral rules is much "easier" than performing utilitarian calculus on individual actions—in this way, rule utilitarianism reduces the cognitive load on imperfect moral agents (read: humans).A central problem with Utilitarianism is that people act based on their perceptions, but the perceptions themselves are based on any individual's sense data, thought-patterns, pre-conceived notions and so on, and thus, they are subjective.