 Headed in the opposite direction, arguments concerning new concepts such as multiple realizability and supervenience, introduced by Putnam, Kim, Fodor and others, have led to talk of higher-level functionalism, a distinction between type-type and token-token reductions and the examination of its implications. The concepts of emergence, supervenience and downward causation are related metaphysical tools for generating and evaluating proposals about unity and reduction in the sciences. This literature has enjoyed its chief sources and developments in general metaphysics and in philosophy of mind and psychology (Davidson 1969; Putnam 1975; Fodor 1975; Kim 1993).