A key binary opposition, underlying much Hindu thought and practice, and maintained by the prescriptions of Dharmaśāstra. It is predicated, in the first instance, on the Brahmanical idea that the twice-born varṇas, and brahmins in particular, possess an inherent ritual purity (śauca, śuddhi) by virtue of their birth. They can, however, be polluted by contact with i) a wide variety of substances, such as bodily fluids, hair, and nail cilippings, leather products, and various foodstuffs (particularly those improperly prepared, or subject to pollution through the relative impurity of the cook), ii) life events, such as birth, death, menstruation, and iii) members of the lower castes, particularly those who are considered avarṇa or ‘untouchable’. Conversely, it is precisely these lower castes which are thought to possess an inherent ritual impurity (aśauca, aśuddhi), and it is to them that are assigned what are thought to be major polluting tasks, such as barbering, laundering, and the disposal of corpses. (In other words, lower castes take on the major pollutions (aśauca) of death, excretion, and other bodily functions for the castes above them.) Change of occupation makes no difference: if a caste is thought of as impure, then its members, regardless of their actual occupations, are thought of as polluting to those above them in the hierarchy. Most such pollutions are temporary, insofar as they dissipate over time, or can be removed through prāyaścitta, and rituals of ‘purification’ (śauca)—typically taking a ‘bath’, since water is seen as a major, ritually purifying agent. This explains the necessity for orthodox brahmins to take a daily bath to maintain their purity: those at the top of the hierarchy being at the same time the most vulnerable to, and the most capable of, absorbing impurity.
The performance of ritual, in particular, requires those involved to purify themselves prior to their involvement. Consequently, those suffering from temporary pollutions, particularly women who are subject to menstruation and childbirth (which in themselves make women inherently less pure than men), are excluded for the duration from activities such as temple-going, or approaching an image. (In many cases this embargo is also applied to the lower castes and mlecchas or foreigners.) An exception to this requirement to maintain ritual purity, which proves and trades on the rule, is the deliberate use of various impure substances (pañca-makāra) in some types of Tantric rite, as a means of acquiring powers (siddhis) through transgression.
Ideas of purity and impurity are extended by analogy to mental or internal processes in (e.g.) yoga, and to ethics, although ritual purity or impurity, with its physical co-requisites, may still be seen by some as the index of an equivalent inner state. In the case of ritual purity, it may, indeed, be seen as a prerequisite of, or preparation for its inner analogue. Others may explicitly deny the connection.