Functionalism may result in attributing minds to complex systems that we otherwise would not consider to have minds. It might result in denying the presence of minds that operate according to different causal principles than our own. Indeed, Hilary Putnam (1926–) himself later rejected functionalism on the grounds that beliefs could not be computational states because their content was determined by external facts, and beliefs were also part of a whole system of knowledge. At the same time as Paul Kripke (1940–) and Keith Donnellan (1931–), he developed a new causal or direct theory of meaning, which was published in The Meaning of “Meaning” (1975).