Taken from An Introduction to Logic by Richard Arthur, page 147:
We come now to what is perhaps the most important rule of inference after modus ponens. Certainly it has been at the creative heart of logic through the years, despite its not being part of the formal logic taught in universities until the twentieth century. As we’ll see below, Galileo Galilei used the reductio ad absurdum argument form to brilliant effect in demolishing the physics of Aristotle, even while pouring scorn on the sterile logic he had learned in the Schools. The reductio is a way of refuting a proposition by showing that it leads to an absurdity, by reducing it to absurdity—hence the Latin name, which translates as “reduction to an absurdity.” Generally, of course, such a reduction cannot be achieved without enlisting the aid of other premises.