Kant's Third Proposition in Groundwork I is this: "Duty is the necessity of an action from respect for law." This paper argues for the textual and philosophical importance of both the adjudicative and motivational components of this proposition. It tries to show that focusing on the law, to the detriment of respect, as is usually done, obscures the problem with which Kant ends Groundwork I and which motivates his transition to a metaphysics of morals in Groundwork II. My paper attempts to remove this obscurity by clarifying the functions of respect and the law in grounding the necessity of duty.