"Epistemology" is the subject matter that deals with truth on a meta level: e.g., questions such as "What is truth?" and "What methods of reasoning are most likely to lead to true beliefs?" Epistemology can be considered as a child subject of Rationality. Many Arbital Discussion Practices have their roots in a thesis about epistemology - about what forms of reasoning are most likely to lead to true beliefs - and you may have ended up looking at this subject after trying to track down how a Discussion Practice is justified.