Popper's demarcation criterion of falsifiability runs into two difficulties. One of which is?

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The difficulty with Popper's demarcation criterion of falsifiability is that scientific theories actually compete with each other as well as with nature. This means that falsification alone cannot provide a clear-cut criterion for determining what is and what is not science, as theories can be amended and refined to better fit with empirical evidence, or even replaced altogether by a rival theory that is better supported by the available evidence.

Popper's Demarcation Criterion of Falsifiability Runs into Two Difficulties