The reason why it is wrong to make a lying promise, according to Kant, is because this act cannot be willed as a general practice without contradiction.

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According to Kant, an action is morally wrong if the rule guiding it cannot be universalized without leading to a contradiction. Making a lying promise would entail willing a world where promises are not trustworthy, which contradicts the very purpose of making promises, thus it cannot be universally willed.

The Reason Why It Is Wrong to Make a Lying