Exchange and reciprocity are central concepts in all forms of human society. Based on a system of mutual obligation, they denote any activity in which valuables are circulated between individuals or groups of people. In the religious sphere they include the transfer of both material and immaterial goods between human and spiritual beings. As outlined by Marcel Mauss in his Essai sur le don, the classic work on the total system of reciprocity, such exchange is governed by the principle of the gift entailing the counter-gift.