Buddhism's main concern is soteriological, defined as freedom from dukkha (unease). Because ignorance of the true nature of things is considered one of the roots of suffering, Buddhist thinkers concerned themselves with philosophical questions related to epistemology and the use of reason. Key Buddhist concepts include the Four Noble Truths, Anatta (not-self) a critique of a fixed personal identity, the transience of all things (Anicca), and a certain skepticism about metaphysical questions. Buddhist thinkers in India and subsequently in East Asia have covered topics as varied as phenomenology, ethics, ontology, epistemology, logic, and philosophy of time.