Counterexample
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A counterexample is an example that is opposite to a theory or argument.
For example, the well-proven theory that cigarettes cause lung cancer has the counterexample of the (rare) individual who might smoke his entire life and not develop lung cancer. The counterexample does not disprove the theory.
A counterexample cannot disprove correlation or causation. An example of flawed liberal logic is the use of a counterexample to argue that a theory is false, as in "I don't go to church and I'm not a bad person, and therefore church is not needed."
A theory of universality, such as the claim that all species evolved, can be disproved by just one counterexample, such as a species (like the whale) that lacks a plausible evolutionary path.