Karl Popper, an Austrian-born, British-based philosopher of science, is widely regarded as one of the most influential philosophers of the latter half of the twentieth century. He is particularly known for his concept of the 'open society' and his work on the philosophy of science. Popper argued that the philosophy of science needed to be fundamentally rethought. A central idea in Popper's philosophy of science is the principle of **falsifiability**, which he proposed as a criterion for demarcating scientific theories from non-scientific ones. According to this principle, for a theory to be considered scientific, it must make predictions that are capable of being shown to be false by observation or experiment. This means a scientific theory must risk being wrong; it must "furnish ample handles for its refutation". If a theory cannot, in principle, be contradicted by empirical evidence, then it is not scientific in Popper's view. Popper emphatically rejected the traditional empiricist model of science, which held that universal truths could be arrived at by induction – inferring general laws from the observation of particular instances. He argued that securing the "verification" or proof of a universal statement based on individual observations is logically impossible. For example, observing a vast number of white swans cannot conclusively prove the hypothesis "All swans are white" to be true. However, a single observation of a black swan is sufficient to prove the hypothesis false. This highlights what Popper saw as a crucial **asymmetry** between verification and falsification. While you can never definitively prove that a theory is true, because it typically makes an infinite number of empirical predictions, you _can_ prove a theory false with just one reliable observation that contradicts it. Based on this principle, Popper repudiated the claims of both psychoanalysis and Marxism to scientific status. He argued that theories like astrology or psychoanalysis avoid subjecting themselves to genuine empirical tests. They might not make precise predictions, or they might arrange their statements in an ad hoc fashion to accommodate empirical results that contradict the theory. For Popper, a pseudoscience attempts to protect a theory from falsification by inventing such ad hoc hypotheses to explain away discrepancies. Popper described scientific progress not in terms of discovering universal truths, but rather as a process of proposing provisional, falsifiable propositions. This led him to propose a model of scientific methodology based on **conjecture and refutation**. In this view, knowledge grows through guesswork (conjectures or tentative solutions to problems) that are then subjected to searching and uncompromising tests (refutations). The emphasis shifts from seeking certainty and justification to focusing on criticism and identifying errors. Scientific theories are entertained only if they are criticizable and, in the realm of empirical facts, empirically falsifiable. Furthermore, Popper argued against the idea of "theory-free," infallible observations, suggesting instead that all observation is **theory-laden**. We see the world through the "distorting glass (and filter) of a preexisting conceptual scheme". This perspective suggests that empirical data are interpretations within the framework of previous theories and therefore share the hypothetical character of those theories. The acceptance of an empirical observation as an instance confirming or testing a hypothesis requires a decision, relying on a pre-understanding that cannot be theoretically elaborated. Popper agreed with this point, suggesting a discursive process within the scientific community similar to jury deliberation is needed. Popper's falsifiability thesis, while influential, has also faced criticism. Critics argue that actual scientific practice does not strictly adhere to Popper's model. When an experiment contradicts a theory, scientists typically ask a host of questions about the experiment, the analysis, the theory itself, or additional assumptions, rather than simply rejecting the theory. The idea that scientific propositions can be tested one by one ("the empiricist dogma") is seen as a "fairy tale" about science. Complex theoretical systems involve core theories and auxiliary hypotheses, and contradictions may lead to adjustments in the latter rather than rejection of the core theory. Some argue that Popper's rigorous philosophical stance on falsifiability, particularly his rejection of induction and confirmation, can inadvertently make science vulnerable to **relativism**. If science cannot "confirm" theories as true but merely maintain them until they are proven false, its claims might seem no more truthful or accurate than other discourses like astrology or myth, especially if the strict criterion of falsifiability isn't always followed in practice. However, Sokal and Bricmont note that while Popper's thesis oversimplifies scientific practice, the emphasis on falsifiability and falsification is still salutary when comparing vastly different endeavors like astronomy and astrology. Popper himself, in later publications, expanded his views and clarified that falsifiability should not be taken as a simple criterion for rationality. Despite the critiques, Popper's emphasis on subjecting theories to critical testing and being open to refutation remains a significant contribution to the understanding of the scientific method. Related concepts in Popper's work include **verisimilitude**, which he was the first to use in philosophy to describe the degree to which a hypothesis approaches the truth, and his self-description as a **critical rationalist**, signifying his skepticism towards philosophies seeking certainty through logic and his emphasis on criticism. His ideas share some common ground with others who emphasized fallibilism and criticism, such as Peirce.