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Universalizability

Maxims lapse into mere subjectivity, and thus become unable to qualify as practical laws, if they produce a contradiction in conception or a contradiction in the will when universalized. A contradiction in conception happens when, if a maxim were to be universalized, it ceases to make coherent sense because the "maxim would necessarily destroy itself as soon as it was made a universal law." For example, if maxims equivalent to 'I will break a promise when doing so secures my advantage' were universalized, no one would trust any promises, so the idea of a promise would become meaningless; the maxim would be self-contradictory because, when universalized, promises cease to be meaningful. The maxim is not moral because it is logically impossible to universalize—we could not conceive of a world where this maxim was universalized.