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Attempts to improve fundamental definitions or classifications are not unique for psychiatry. In a 'hard science' such as metrology—the discipline of measurements in the natural sciences—a major definitional change has been proposed. In 2019, the kilogram will be redefined in terms of a constant of nature instead of a cylinder of platinum-iridium. In this article, we aim to better understand the reasons and procedures for changing definitions by studying the redefinition of the kilogram. In short, the case of the kilogram shows that 1) sometimes the rationale for a redefinition might be clear but the scientific discoveries are not available, and a vast research effort is needed to make progress, 2) progress can be made even in absence of gold standards by reference to the definitions' epistemic aims, and 3) definitions are unlikely to be final as future discoveries might lead to new definitions. These results support the current approach of ongoing, piecemeal revision of psychiatric disease classifications, and stress the importance of robust scientific evidence before changing definitions.