"Kill one to save five?"
A runaway trolley is barreling down a track towards five tied-up workers. You stand by a lever. If you pull it, the trolley switches to a side track where only one person is tied up. Do you pull the lever? You have two choices: kill one person or let five die.
Philosophers like Jeremy Bentham would argue for Utilitarianism: the best action is the one that maximizes overall happiness. 5 lives > 1 life. Therefore, you represent a moral obligation to pull the lever. The math dictates the morality.
Immanuel Kant's Deontology argues that morality is based on rules and duties. Killing an innocent person is inherently wrong, regardless of the outcome. By pulling the lever, you become an agent of death. Therefore, you must not intervene. Humans should be treated as ends, never as means.