The question is not whether one can formally describe what Ann knows and believes under the assumption that she considers it impossible that her opponent will choose action \(a\). Indeed, an epistemic-probability model where Ann assigns probability zero to the event that her opponent chooses action \(a\) is a perfectly good description of Ann’s epistemic state. The problem is that this model blurs an important distinction between Ann being unaware that action \(a\) is a live possibility and Ann ruling out that action \(a\) is a viable option for her opponent. This distinction is illustrated by the following snippet from the well-known Sherlock Holmes’ short story Silver Blaze (Doyle 1894):