Imagine the following scenario: There are five people on a train track with a train rapidly approaching. The only way to stop the train is to put something very heavy in front of it. As it happens, there is a very fat man on a bridge right above the train. If you push the fat man in front of the train, you save the five people on the tracks, but obviously will kill the fat man.
What do you choose--to push the fat man or to do nothing?
Some people say that they would push the fat man. They conclude that five lives are of greater value than one and therefore that killing one man to save five lives is just. Essentially, this line of reasoning is an appeal to Utilitarianism, a philosophy associated with John Stuart Mill. Utilitarianism holds that we can assess the value of an action based on the extent to which it furthers utility--the greatest good for the greatest number. In the case at hand, the lives of five human serves the greatest good for the greatest number. Utilitarianism dictates that the fat man should be pushed.
Pushing the fat man also relies upon a concept known as consequentialism. Consequentialism simply means that actions are to be judged on their effects, their consequences. Utilitarianism is a form of consequentialism because it focuses upon whether the consequence of an action is the promotion of utility. Since the death of one man is less of a grave consequence than the death of five, consequentialist reasoning leads to the conclusion that the fat man should be pushed. Philosophers often call the consequence of an action the "end" and the action itself the "means".
According to Mill, we ought to do that action that allows us to maximize goodness, or happiness.
Mill is very concerned about preserving individual freedom, and the conditions in which we choose to restrict personal autonomy. When you hear about Mill’s about liberty you will hear the following terms:
harm principle: you cannot violate someone’s freedom unless they have caused harm to another person
paternalism: one person restricting someone’s freedom in a “parent” role, “for their own good”
marketplace of ideas: shorthand for Mill’s idea that we need a diversity of opinions/ideas in order to form the best conclusion possible
the “truth”: the ultimate goal of our behaviors, find out what is valid
utility: do what maximizes the “greatest good for the greatest number”