Who was Karl Popper? Karl Popper (1902–1994) was an Austrian-British philosopher of science, widely regarded as one of the most influential thinkers of the twentieth century. Born in Vienna, he fled the Nazi threat and settled in New Zealand before moving to London, where he taught at the London School of Economics. His major work, The Logic of Scientific Discovery, challenged prevailing views about how science works. Popper argued that scientific theories cannot be proven true but can be falsified, and he made falsifiability the criterion for distinguishing science from pseudoscience. He also wrote on politics, defending open society and criticizing totalitarianism.

What is Popper's falsifiability criterion? Popper argued that the defining feature of a scientific theory is not that it can be proven true, but that it can be proven false. A theory is scientific if it makes predictions that could, in principle, be shown to be wrong by observation or experiment. If no possible observation could refute a theory, then it is not scientific. This criterion distinguishes science from pseudoscience, such as astrology or certain forms of psychoanalysis, which often explain away any evidence. Falsifiability does not mean that a theory is false; it means that the theory is testable and risky.

Why did Popper reject induction? Popper rejected the idea that science proceeds by induction, that is, by accumulating observations and generalizing from them. He argued that no number of observations can ever prove a universal statement. Observing a million white swans does not prove that all swans are white, because a single black swan would refute it. Instead, Popper argued that science proceeds by conjecture and refutation: scientists propose bold hypotheses and then try to falsify them. Knowledge grows not by confirming theories but by eliminating errors. This approach is known as critical rationalism.

What did Popper mean by conjecture and refutation? Conjecture and refutation is Popper's description of the scientific method. Scientists begin by proposing bold conjectures, imaginative hypotheses about how the world works. They then subject these conjectures to severe tests, trying to refute them. If a conjecture survives testing, it is tentatively accepted, but never proven. If it fails, it is rejected and replaced by a new conjecture. This cycle of conjecture and refutation drives scientific progress. Knowledge grows not by accumulation but by the elimination of errors. For Popper, this process is the essence of rationality.

What is the problem of demarcation? The problem of demarcation is the question of how to distinguish science from non-science or pseudoscience. Popper proposed falsifiability as the criterion: a theory is scientific if it makes testable predictions that could be refuted. Other philosophers have proposed different criteria, such as Kuhn's notion of normal science or Lakatos's research programs. The problem is important because it affects how we evaluate claims about the world, from climate change to alternative medicine. While no single criterion is universally accepted, the debate has clarified what is at stake in calling something scientific.

What did Popper mean by the open society? In his political philosophy, especially in The Open Society and Its Enemies, Popper defended liberal democracy against totalitarianism. An open society is one in which individuals are free to criticize their leaders, propose new ideas, and change the rules through peaceful means. It contrasts with a closed society, in which authority is absolute and criticism is suppressed. Popper argued that open societies are superior because they allow for the correction of errors and the peaceful resolution of conflicts. His defense of the open society was influenced by his experiences of fascism and communism.

How did Popper view Marxism and psychoanalysis? Popper criticized both Marxism and psychoanalysis as pseudosciences. He argued that their theories were not falsifiable because they could explain any outcome. For example, a Marxist might explain a revolution as the inevitable result of class conflict, and a lack of revolution as false consciousness. Similarly, a psychoanalyst might explain any behavior as the result of unconscious drives. Because these theories could accommodate any evidence, Popper argued that they lacked the risky predictions that characterize genuine science. His critique was controversial but influential in debates about what counts as science.

What is Popper's concept of verisimilitude? Verisimilitude is Popper's term for truthlikeness, the idea that a theory can be closer to the truth than another even if both are false. Since scientific theories are always conjectural and can never be proven true, Popper needed a way to compare them. He suggested that a theory has greater verisimilitude if it makes more true predictions and fewer false ones than its rivals. The concept has been widely discussed and criticized, but it captures an important intuition: science progresses by getting closer to the truth, even if it never fully arrives.

How did Popper influence later philosophy of science? Popper's ideas have been enormously influential in the philosophy of science. His emphasis on falsifiability and critical testing shaped debates about scientific method. His critique of induction inspired alternative accounts, such as Bayesian confirmation theory and inference to the best explanation. His political philosophy influenced defenders of liberal democracy and open society. Thinkers such as Imre Lakatos, Paul Feyerabend, and Thomas Kuhn developed their views in dialogue with Popper, sometimes in opposition. His insistence on criticism and fallibility remains central to the philosophy of science today.

What is Popper's legacy? Karl Popper's legacy is broad and enduring. In the philosophy of science, he is remembered for falsifiability, critical rationalism, and the critique of induction. In political philosophy, he is remembered for his defense of the open society and his critique of totalitarianism. His ideas have been applied in fields as diverse as economics, psychology, and education. He was a public intellectual who engaged with the issues of his time and defended reason, freedom, and criticism. His work continues to be studied and debated, and his questions about science and society remain as relevant as ever.