In this paper, I propose to differentiate two types of presumptions in law and in legal argumentation. On the one hand, the so-called hominis presumptions, that is, those made by people when they make factual inferences and, on the other, the presumptions established by legal norms (legal presumptions). In order to emphasize the differences between them, I will use these two expressions respectively: “it is presumable” and “it must (shall) be presumed”. Next, once the notion of legal presumption has been properly clarified, I will try to show that the distinction between rules and principles is applicable to presumption norms (to legal presumptions). Consequently, I will distinguish between norms of presumption that are rules (presumption rules) and norms of presumption that are principles (presumption principles). Finally, I will focus on the defeasibility of presumptive reasoning and how cognitive sciences can help detecting material fallacies.