Distressed by his own personal tragedies, the Roman philosopher Cicero once asked himself whether a wise person should try to achieve the Stoic ideal of complete emotionlessness. Cicero reasoned that, however desirable the goal may be, a wise person could never attain it, since emotions are not simply irrational urges. They are, rather, a product of one's estimate of the goodness and badness of the events, people, and actions one witnesses.
Which of the following is an assumption required by Cicero's reasoning?