Constitutive arguments for the principles of practical reason attempt to justify normative requirements by claiming that we already accept them in so far as we are believers or agents. In two constitutive arguments for the requirement that we must will universally, Korsgaard attempts first to arrive at the requirement that we will universally from observations about the causality of the will, and secondly to establish that willing universally is constitutive of having a self. Some rational requirements may be established by some version of this second argument, but the strategy does not seem promising when it comes to establishing the requirement that we will universally. I draw on the discussion of Korsgaard to highlight a challenge facing constitutive arguments in general.