Final answer:
Utilitarianism is the ethical principle that justifies claims about the morality of an act with one good effect and one harmful effect. It weighs the balance of overall consequences to determine moral permissibility.
Step-by-step explanation:
The ethical principle that is invoked to justify claims that a single act, which has one good effect and one harmful effect, is not always morally prohibited is utilitarianism. Utilitarianism is a consequentialist ethical theory that judges the morality of an action based on the overall consequences it produces, with the aim of maximizing happiness or well-being for the greatest number of people.
For example, in the case of the trolley problem, where a person must decide to divert a runaway trolley onto a track with one person or let it continue on a track with five people, a utilitarian would choose to divert the trolley to minimize overall harm and save more lives.
Overall, utilitarianism considers the consequences of an action and weighs the balance of good and harm to determine its moral permissibility.