Popper's falsifiability is a principle proposed by philosopher Karl Popper that suggests a theory is scientific only if it can be tested and potentially proven false. This means that for a theory to be considered valid, there must be a possible observation or experiment that could contradict it. For example, the statement "All swans are white" is falsifiable because discovering a single black swan would disprove it. In contrast, theories that cannot be tested or potentially disproven, such as those based on personal beliefs or vague assertions, do not meet the criteria of scientific inquiry according to Popper.