Such nonreductive materialists include functionalists about the mind, who hold that psychological properties differ from material properties in virtue of the special causal or functional roles of the former. Functionalists differ from behaviorists in acknowledging the psychological relevance of causal relations among not only stimuli and behavior but internal states as well. According to many functionalists, the causal roles that determine psychological properties are specified by the taxonomies, or systems of classification, found in the best contemporary psychology. These causal roles can depend on relational considerations that are independent of considerations about the composition of what exemplifies psychological properties. Thinking, according to functionalism, does not require a specific physical composition for thinkers.
Physical composition can vary as long as the appropriate causal-relational features obtain.
Nonreductive Materialism Nonreductive materialists generally follow functionalists in emphasizing the importance of multiple realizability. Before we can appreciate the evidence for multiple realizability, we need an appropriate vocabulary. An egg, for example, is a type of single-celled organism. These individual eggs are tokens . A particular mastiff may be a token of the type mastiff , but it is also a token of the type dog , mammal , animal , domesticated animal , and slobbering thing .