Most Kantian constructivists try to ground universal duties of interpersonal morality in certain interactions between individuals, such as communication, argumentation, shared action or the second-person standpoint. The goal of this paper is to present these, which I refer to as arguments from the second-person perspective, with a dilemma: either the specific kind of interaction that is taken as a starting point of these arguments is inescapable, but in that case the argument does not justify a universal principle of interpersonal morality. (...) Or interaction does have a principle of interpersonal morality among its necessary conditions of possibility, but such forms of interactions are merely optional. I argue that proponents of arguments from the second-person perspective have failed to provide a convincing response to this dilemma and that this failure is systematic. This suggests that the success of Kantian constructivism depends on the success of arguments from the first person. (shrink)