Type-type identity theory is so called because it identifies mental and physical types or properties on a par with identifying the property of being water with the property of being composed of H2O molecules. After a brief period of popularity in the early days of contemporary physicalism during the 1950s and 60s (Place 1956, Smart 1959) it has been far less widely held because of problems such as the multiple realization objection according to which mental properties are more abstract and thus