What distinguishes a scientific method of inquiry is a question known as 'the criterion'. It is an answer to the question: is there a way to tell whether a concept or theory is science, as opposed to some other kind of knowledge or belief? There have been many ideas as to how it should be expressed. Logical positivists thought a theory was scientific if it could be verified; but Karl Popper thought this was a mistake. He thought a theory was not scientific unless there was some way it might be refuted. On the other hand, Paul Feyerabend thought there was no criterion. For him, "anything goes", or whatever works, works.