Abstract. Epistemic logic pays barely any attention to the notion of understanding, and stands in total contrast to the current situation in epistemology and in philosophy of science. This presentation will focus on understanding why in an epistemic-logic-style. It is generally acknowledged that understanding why moves beyond knowing why. Inspired by philosophical ideas, we consider whereas knowing why requires knowing horizontal explanations, understanding why additionally requires vertical explanations. Based on justification logic and existing logical work for knowing why, we build up a framework by introducing vertical explanations, and show it could accommodate different philosophical viewpoints via adding further conditions on the models. Note that notions of explanation and justification will be connected in our framework, which has an interesting consequence that understanding why is necessary for knowing why in epistemic situations. A sound and complete axiomatization for the most general case is given, and we leave other particular axiomatizations corresponding to certain philosophical views to a future occasion.
