Perhaps the most emotionally charged thought experiments appear in moral philosophy. is a famous response to anti-abortion arguments. She asks: What if you wake up to find yourself attached, without your consent, to a famous unconscious violinist whose survival depends on your kidneys for nine months? Are you morally obligated to stay attached? Most people say no. Thomson uses this analogy to argue that even if a fetus is a person with a right to life, that right does not automatically override the pregnant person’s right to bodily autonomy. The thought experiment does not settle the abortion debate, but it reframes it, exposing a hidden assumption that “right to life” means “right to use another’s body without consent.”

In epistemology—the study of knowledge—few thought experiments are as powerful as or its modern successor, Hilary Putnam’s Brain in a Vat . Descartes asks: What if an all-powerful evil demon is deceiving me about every single thing I perceive? The sky, my body, mathematics—all could be illusions. This radical doubt is not meant to paralyze us but to locate an indestructible foundation for knowledge: “I think, therefore I am.” Putnam updates the scenario: What if you are a brain floating in a vat of nutrients, wired to a supercomputer that simulates reality? Could you ever know you are not a brain in a vat? The “what if” here reveals a fracture in naive realism and forces philosophers to confront skepticism not as a joke, but as a serious logical possibility that any robust theory of knowledge must address.

One classic example from such a collection is . The scenario asks: What if a shepherd found a ring that made him invisible? If no one could see you commit a crime, would you still be just? Plato uses this thought experiment to challenge the view that morality is merely a social contract. He argues that a truly just person would not use the ring, even with impunity—not because of fear of punishment, but because justice is an intrinsic good. The “what if” strips away external consequences and forces us to examine the soul’s inner character. This thought experiment has echoed through centuries, influencing debates in ethics, law, and psychology.