Utilitarianism is probably the most commonly used moral philosophy in modern Western Society, but so often people will exclusively use the form of “Act” utilitarianism as opposed to “Rule” utilitarianism. Jeremy Bentham, the originator of utilitarianism, defines it in brief with his phrase, "the greatest happiness of the greatest number". When you hear a leftist implore this phrase, they are falling prey to the weakness of Act utilitarianism and embrace an ad hoc approach to running our country.
Wikipedia does a good job of explaining the difference between “Act” and “Rule” Utilitarianism.
“Act utilitarianism states that, when faced with a choice, we must first consider the likely consequences of potential actions and, from that, choose to do what we believe will generate most pleasure. The rule utilitarian, on the other hand, begins by looking at potential rules of action. To determine whether a rule should be followed, he looks at what would happen if it were constantly followed.”
I do not believe in the morality of utilitarianism, but it does accurately describe how people behave. I hold it as a matter observed fact that when presented with a choice, humans will try to calculate their self-interest (Act Utilitarianism) and/or rely on a pre-established rule (Rule Utilitarianism). Much of modern economics is based on this premise.
In our personal lives we run into situations where we must make, what many would call, a moral decision. Say, you have not started on a project that your boss is expecting you to finish and he/she has come by to ask about your progress. Should you lie or should you admit your progress and assure him/her that it will be finished on time?
The Act utilitarian would mull whether he thought he could get away with the lie. Because of our limited faculties, this is a difficult calculation. Might the boss immediately ask, “Can I see what you have so far?”, or will he naively believe you. We possess an arrogance that we can calculate these events with high accuracy. This is why policemen have an enormous advantage when questioning a suspect. They have the resources to check all the available facts, yet the suspect must rely on his memory to formulate an alibi without contradiction to reality.
The Rule utilitarian would have decided beforehand whether he would lie. Perhaps he would decide never to lie to anyone. By choosing this, he is knowingly giving up the opportunity to gain from lying, but assuring that he will not suffer the consequences of being caught. In making this choice, the rule utilitarian can rely on a broad history of his own experiences and the experiences of others. He is using a large amount of information, whereas the Act utilitarian is using a very limited information set.
Libertarians are labeled ideologues because we quickly reject ideas when we hear that which does not fit the rules that we have developed around our understanding of how the world works. This belittlement is akin to the thinking of teenagers and college students who chafe at parental suggestions of appropriate behavior. They arrogantly believe that they can enjoy the fruits of licentious behavior and avoid the fateful consequences. If the calculation of Act Utilitarianism is so difficult for an individual, how could one reasonably believe that the calculation of coercion over millions of individuals in aggregate would be anything but impossible?
Unfortunately, our leaders are drunk with this conceit. The tendency now is to eschew principles and toss aside careful deliberation, as the new President said at his inauguration, “the state of the economy calls for action, bold and swift.” Hundred billion dollar “fixes” come across the newswires every few days. Warnings of graft, unintended side effects, and futility are mocked as fears born of “worn-out dogmas.” The calculations one needs to perform Act utilitarianism through government planning number in the trillions of trillions, but haste cannot afford to be troubled with reality. We charge forth into battle, assured that mere courage will overcome the entrenched and daunting reality.