Logik der Forschung is a 1934 book by Karl Popper. It was originally written in German, but reformulated in English by Popper himself some years later, to be published as The Logic of Scientific Discovery in 1959. This forms the rare case of a major work to appear in two languages, both written by the author instead of being translated. Sir Peter Medawar called it "one of the most important documents of the twentieth century." In it, Popper argued that science should adopt a methodology based on falsification, because no number of experiments can ever prove a theory, but a single experiment can contradict one. Popper holds that empirical theories are characterized by falsifiability.