In "A Defense of Abortion," Judith Jarvis Thomson asks us to imagine that we are kidnapped and drugged, and awake to find ourselves with tubes connecting us to an unconscious stranger. Upon inquiry, we discover that the stranger is a world famous violinist, and that the tubes connecting us to the violinist are the only thing keeping the violinist alive. Should we disconnect, the violinist will surely die. If we remain connected for nine months, the violinist will awaken and be able to survive on their own.
Peter Singer's "Famine, Affluence and Morality" asks whether you would have a moral obligation to save a drowning child if the only risk to yourself were that you would ruin your new clothes. Singer suggests that the underlying principle behind our intution that, yes, of course we should save the child at the cost of our clothing is that if I can prevent harm to others without causing any harm of comparable moral significance, then I ought to do it.
René Descartes posits the existence of an evil demon in Meditations on First Philosophy. Descartes suggests that he may not be able to trust the evidence of his own senses because, for all he knows, an evil demon could somehow be deceiving him about what he seems to be experiencing. He concludes that he must doubt the existence of everything around him.
"Killing, Letting Die and the Trolley Problem" tackles what has become one of the best-known moral philosophy problems around, thanks to several rounds on Twitter and a full episode of "The Good Place." This version, also from Thomson, finds you on a runaway trolley approaching a switch. If you do nothing, your trolley will run over five innocent people on the tracks. If you activate the switch, you will divert the trolley onto a side track, where you will run over one innocent person.
The unconscious violinist is meant to justify abortion in cases of rape. Thomson argues that the violinist is innocent of any wrongdoing, but he nevertheless has no right to the use of your body for nine months following an action to which you did not consent. So, too, is the case of a fetus who enters a woman's body without her consent.
Singer's essay is an argument for the moral theory of utilitarianism, which holds that we have an obligation to maximize the amount of good in the world. He argues that many of our purchases—while they do produce happiness for us—do not produce nearly as much happiness as one gets from not starving to death. Singer concludes that utilitarianism commits us to selling many of our possessions and using the proceeds to prevent others from dying of starvation, illnesses, and so forth.
Descartes invokes the evil demon as part of a project to build up a theory of knowledge that is grounded in certainty. He invokes various examples to show why he could doubt many things, until he finds one thing about which he is certain—the famous Cogito ergo sum, or "I think, therefore I am." The one thing about which Descartes cannot be fooled is the fact that he exists as the thing that is being fooled.
The trolley problem presents a challenge to utilitarianism. For someone like Singer, the solution to the trolley problem is simple: throw the switch and kill one person instead of five. Accounts of morality that value personal choice over the greater good suggest that throwing the switch makes you morally culpable for the death of one person.
These kinds of conceptual metaphors are vital for understanding new or complex topics. It’s why philosophy is full of unconscious violinists, children drowning in ponds, evil deceivers and runaway trolleys.