Falsifiable
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A proposal or theory is falsifiable if it can be shown to be false when it is false. A proposal or theory that cannot never be shown to false even though it is false is not falsifiable.
For example, the suggestion that I would be a very happy if I were ten years younger is not falsifiable, and thus not worth thinking about. We have no way of knowing if it is false.
Karl Popper persuasively argued that a theory that is not falsifiable is not scientific, because there is no possible way to disprove it. This is known as the doctrine of falsification.