Galileo probably settled falling bodies with logic, not the Leaning Tower
The familiar picture has Galileo dropping two weights from the Leaning Tower of Pisa. His own 1638 book tells a different story: he proved that heavy and light bodies fall at the same rate with pure reasoning, by imagining two stones tied together. It is one of history's great thought experiments.
A thought experiment is an imagined scenario built to clarify or test an idea, often because the real version would be impractical, impossible or unethical. It may also be an abstract hypothetical aimed at our intuitions about ethics or other deep questions. The ancient Greeks already used the approach for conceptual proofs in mathematics, and philosophers were posing structured what-if questions even before Socrates.
Galileo's argument, voiced by the character Salviati in Two New Sciences, runs like this. Suppose heavier bodies fall faster. Tie a fast, heavy stone to a slower, lighter one: the slow stone should drag on the fast one, so the pair falls more slowly than the heavy stone alone. But the tied pair is heavier than either stone, so by the original assumption it should fall faster. The premise contradicts itself. Nothing new was measured; familiar experience was simply rearranged to yield a fresh inference.
The name arrived much later. According to Johann Witt-Hansen, Hans Christian Ørsted first used the German Gedankenexperiment around 1812. By 1883 Ernst Mach was using it in a narrower sense, for mentally rehearsing an experiment his students would then perform for real; when the two results differed, he wanted an explanation. The English phrase first appeared in an 1897 translation of one of Mach's papers.
Physics is full of famous examples, many from the 19th and 20th centuries. Schrödinger's cat, apparently alive and dead at once, was designed to expose absurd consequences of the Copenhagen Interpretation of quantum mechanics, while Maxwell's demon imagines a small being that seems to break the Second Law of Thermodynamics. The method now turns up in ethics, economics, law, where it is often called a hypothetical, epidemiology and science fiction.
Source: Thought experiment
