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Falsifiability Deductive arguments could be said to be like computer programs — the conclusions they reach are only as good as the data that is fed into them.

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Deductive reasoning has an important role to play in the sciences, but on its own, it cannot say anything about the world.

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It can only say “If this is the case, then that is the case.”

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And if we want to use such arguments in the sciences, we still have to rely on induction for our premises, and so science is lumbered with the problem of induction.

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For this reason, according to Popper, we cannot prove our theories to be true.

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Moreover, what makes a theory scientific is not that it can be proved at all, but that it can be tested against reality and shown to be potentially false.

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In other words, a falsifiable theory is not a theory that is false, but one that can only be shown to be false by observation.

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Theories that are untestable (for example, that we each have an invisible spirit guide, or that God created the universe) are not part of the natural sciences.

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This does not mean that they are worthless, only that they are not the kinds of theories that the sciences deal with.

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The idea of falsifiability does not mean we are unjustified in having a belief in theories that cannot be falsified.

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Beliefs that stand up to repeated testing, and that resist our attempts at falsification, can be taken to be reliable.

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But even the best theories are always open to the possibility that a new result will show them to be false.

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Popper’s work has not been without its critics.

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Some scientists claim that he presents an idealized view of how they go about their work, and that science is practiced very differently from how Popper suggests.

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Nevertheless, his idea of falsifiability is still used in distinguishing between scientific and non–scientific claims, and Popper remains perhaps the most important philosopher of science of the 20th century.