Although today we tend to invoke supermajority rules as an alternative to majority rule, the classical medieval justification for supermajority rules emerged as a substitute for unanimity. As the preceding chapter suggested, the unanimity requirement for the election of the pope broke down early in the second millennium, and was replaced with the doctrine of the sanior et maior pars, by which those electors possessing sounder judgment prevailed. But conflict over the criteria for sound judgment led again to breakdown. Further, following innovations designed to improve the learning and moral quality of the cardinals who served as electors, a strictly aggregative procedure could be introduced: each elector had a judgment worthy of being counted. In choosing a decision rule, the aim was to reflect the epistemic dignity of the electors through a strictly aggregative mechanism.

Supermajority rule from the twelfth century reflected commitments that made it in many ways more attractive than the unanimity it replaced. The most important of these commitments was the acceptance of fallibility, both moral and epistemic. Because people can err both in conduct and judgment, a unanimity rule came to be seen as excessively demanding. A unanimous election required perfect concord: no member could veto a candidate for his own private purposes, or fail to correctly identify the pope. In contrast, a supermajority rule accommodated the possibility that members of an assembly would fail to recognize the truth or would not agree, but wide support of the electors signified both that the outcome reflected the divine will and would command the cardinals’ allegiance.