One of the first thinkers to criticize logical positivism was Karl Popper. He advanced falsification in lieu of the logical positivist idea of verificationism. Falsificationism argues that it is impossible to verify that beliefs about universals or unobservables are true, though it is possible to reject false beliefs if they are phrased in a way amenable to falsification. Thomas Kuhn's idea of paradigm shifts offers a broader critique of logical positivism, arguing that it is not simply individual theories but whole worldviews that must occasionally shift in response to evidence.