Popper and other twentieth-century philosophers of science focused on method. They claimed that all of the sciences, as opposed to other fields of inquiry, shared a common methodology that deserves the name of "the scientific method." Philosophers disagreed about what this method was, but no one doubted that a single scientific method characterized all of science from the time of the scientific revolution, which they also saw as a single event. According to inductivists, scientists must find data that support a hypothesis or theory. Testing that provides positive results confirms a scientific theory and shows how it is grounded in reality. Thus, in the inductivist model of science, pseudoscience is a system of beliefs unconfirmed by experimental data. On the other hand, Popper and other supporters of the hypothetico-deductive method reject the idea of confirming theories, since there are always alternative hypotheses that can account for the same data and thus can be just as well confirmed. They argue instead that scientists formulate hypotheses and then test them by making predictions on the basis of the hypotheses and checking the validity of their predictions. If the predictions are inaccurate, the hypothesis must be rejected; if they are accurate, the hypothesis is accepted, but only tentatively as a hypothesis that has withstood the test and has not yet been falsified. According to Popper, the mark of a scientist is his or her willingness to subject a hypothesis to refutation. Scientific theories are those in which refuting instances can be specified. Pseudoscience, by contrast, is not refutable, since its practitioners construct an explanation for any data that seem to contradict their beliefs.
The notion that there is a single scientific method that defines science has been severely challenged, especially by historians and sociologists of science. Thomas Kuhn famously argued that each domain and period of science is defined by a "paradigm" that sets up problems to be solved and the methods for doing so. However, we find that the sciences are a diverse and disunified collection of practices and that there is no uniform scientific method common to all paradigms, which makes the task of finding a single definition that defines science and separates it from pseudoscience seem impossible.