J. Madeira Nogueira and M. A. Asano de Medeiros, “VALUING THINGS THAT DO NOT HAVE VALUE. EXISTENCE VALUE, ECONOMICS AND ENVIRONMENT”., CC&T, vol. 16, no. 3, pp. 59–83, Jan. 1999, doi: 10.35977/0104-1096.cct1999.v16.8905.