In the First Section of the Groundwork, Kant argues that a good-willed person “under subjective limitations and hindrances” is required “never to act except in such a way that [she] could also will that [her] maxim should become a universal law.” Call this argument “K”. Although recent commentators (including Barbara Herman, Christine Korsgaard, Nelson Potter, and Allen Wood) have done much to clarify and defend many of the important claims Kant makes in the First Section, they have accurately identified neither K's premises nor the reasoning by means of which K's conclusion is derived. The result of this is that K's strengths are underappreciated. My aim is to rectify this state of affairs, by providing a detailed reconstruction of K, and thereby bring out the various ways in which the argument deserves our recognition and praise.