“Of the three great skeptics I interviewed, Popper was the first to make his mark. His philosophy stemmed from his effort to distinguish pseudoscience, such as Marxism or astrology or Freudian psychology, from genuine science, such as Einstein's theory of relativity. The latter, Popper decided, was testable; it made predictions about the world that could be empirically checked. The logical positivists had said as much. But Popper denied the positivist assertion that scientists can prove a theory through induction, or repeated empirical tests or observations. One never knows if one's observations have been sufficient; the next observation might contradict all that preceded it. Observations can never prove a theory but can only disprove, or falsify it. Popper often bragged that he had "killed" logical positivism with this argument.”

Source: The End of Science (1996), p. 27