Yoga is an ancient philosophical system from South Asia that distinguishes between two kinds of explanation: personal explanation, founded on choice and responsibility; and natural explanation, which is causal explanation from the outside. When we are well, we are in charge of our lives, and this means that we order the natural aspects of our life to express our personal freedom. When we are unwell, the natural predominates. Nature, or prakṛti, is accordingly divided into three characteristics or guṇas: sattva (the illuminating, calm and social), rajas (the active and agitated), and tamas (the inert). The practice of Yoga reclaims our life from being governed by the guṇas. In this case, choose what state to be in. Recently, the neuroscientist Dr. Stephen Porges (Professor of Psychiatry at the University of North Carolina at Chapel Hill) developed an account of the functioning of the autonomic nervous system that mirrors Yoga's guṇa theory, but also augments the received view of the autonomic nervous system. The traditional view is that that there are two modes to the nervous system. One is the sympathetic system (the rajasic, concerned with fight or flight) and the parasympathetic (the tamasic), which operates through the Vegas nerve. Polyvagal theory claims that there are two different modes or functions of the vagal nerve. One is the tamassic (the dorsal vagal), while the other corresponds to the sattvic (the ventral vagal, which promotes feelings of safety in a social world). According to Polyvagal theory, we need to cycle through these various states in response to various life needs. But when we are ill, this system can get stuck in the sympathetic (rajassic) or dorsal vagal mode of the parasympathetic system (the tamassic). Then people cannot function.