 Here is a way to understand the tracking requirement. Suppose that, although an agent maintains a latent probabilistic belief state, most of her cognitive life is spent reasoning with and updating qualitative beliefs. A typical day will go by without having to engage the probabilistic system at all. Suppose Monday is a typical day. Let \(\langle \alpha(\Pr), \Pr \rangle\) be the belief state she woke up with on Monday: her full and partial beliefs are in harmony. Let \(E\) be the total information she acquired since waking up. Since qualitative beliefs are updated on the fly, she goes to sleep with the qualitative belief state \(\alpha(\Pr)_E\). Overnight, her probabilistic system does the difficult work of Bayesian conditioning and computes the partial belief state \(\Pr_E\), just in case she runs into any sophisticated decision problems on Tuesday. Before waking, she transitions out of her probabilistic system \(\Pr_E\) and into the qualitative belief state \(\alpha(\Pr_E)\). If she fails the tracking requirement, she may wake up on Tuesday morning with a qualitative belief state that is drastically different from the one she had went to sleep with on Monday night. If she tracks, then she will notice no difference at all. For such an agent, no mechanism (other than memory) is required to bring her full and partial beliefs back into harmony on Tuesday morning. Supposing that we  enter the probabilistic system by conditioning our previous partial belief state \(\Pr\) on all new information \(E\), and  exit by accepting \(\alpha(\Pr_E),\) tracking ensures that transitioning in and out of the probabilistic system does not induce any drastic changes in qualitative beliefs. An agent that tracks will notice no difference at all. An agent that does not track may find her full and partial beliefs perpetually falling out of sync, requiring many expensive acceptance operations to bring them back into harmony. 