The 20th-century Austrian-British philosopher Karl Popper believed that any theory that was not ‘falsifiable’ – capable of being tested and proven incorrect – should be dismissed as unscientific. He was particularly critical of Marxist theory, which he believed was constantly being revised by its adherents to account for its failed predictions, and therefore could not possibly be scientific. The falsification principle is a cornerstone of the modern scientific method, but some contemporary scientists, cosmologists and philosophers believe it might need to be revised as they investigate concepts such as string theory and the multiverse, which come up against the limits of what is testable – at least for now.