Brink, David, "Mill's Moral and Political Philosophy"
The Stanford Encyclopedia of Philosophy (Fall 2014 Edition)
Edward N. Zalta (ed.), forthcoming
Here is an excerpt:
2.6 Utilitarianism as a Standard of Conduct
We might expect a utilitarian to apply the utilitarian principle in her deliberations. Consider act utilitarianism. We might expect such a utilitarian to be motivated by pure disinterested benevolence and to deliberate by calculating expected utility. But it is a practical question how to reason or be motivated, and act utilitarianism implies that this practical question, like all practical questions, is correctly answered by what would maximize utility. Utilitarian calculation is time-consuming and often unreliable or subject to bias and distortion. For such reasons, we may better approximate the utilitarian standard if we don't always try to approximate it. Mill says that to suppose that one must always consciously employ the utilitarian principle in making decisions
… is to mistake the very meaning of a standard of morals and confound the rule of action with the motive of it. It is the business of ethics to tell us what are our duties, or by what test we may know them; but no system of ethics requires that the sole motive of all we do shall be a feeling of duty; on the contrary, ninety-nine hundredths of all our actions are done from other motives, and rightly so done if the rule of duty does not condemn them. (U II 18)
Later utilitarians, such as Sidgwick, have made essentially the same point, insisting that utilitarianism provides a standard of right action, not necessarily a decision procedure (Methods 413).
If utilitarianism is itself the standard of right conduct, not a decision procedure, then what sort of decision procedure should the utilitarian endorse, and what role should the principle of utility play in moral reasoning? As we will see, Mill thinks that much moral reasoning should be governed by secondary precepts or principles about such things as fidelity, fair play, and honesty that make no direct reference to utility but whose general observance does promote utility. These secondary principles should be set aside in favor of direct appeals to the utilitarian first principle in cases in which adherence to the secondary precept would have obviously inferior consequences or in which such secondary principles conflict (U II 19, 24–25).
The entire entry is here.