According to an influential hypothesis, the speech act of assertion is subject to a single 'constitutive' rule, that takes the form: "One must: assert that p only if p has C". Scholars working on assertion interpret the assumption that this rule is 'constitutive' in different ways. This disagreement, often unacknowledged, threatens the foundations of the philosophical debate on assertion. This paper reviews different interpretations of the claim that assertion is governed by a constitutive rule. It argues that once we understand the full import of assuming that assertion is governed by a constitutive rule, it becomes clear that some fundamental assumptions of the current debate are mistaken, and others unwarranted.