Why, for Kant, can't we be sure the grocer is acting from duty when she charges an honest price?

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Kant argues that an action can only be considered truly moral if it is done from duty, not merely in accordance with duty. If an action, like charging an honest price, also aligns with the grocer's self-interest (e.g., maintaining a good reputation), then we cannot be sure the action is motivated purely by duty rather than by a coincidental alignment of duty and inclination.

Why, for Kant, Can't We Be Sure the Grocer Is