  Materialist views say that, despite appearances to the contrary, mental states are just physical states. Behaviourism, functionalism, mind-brain identity theory and the computational theory of mind are examples of how materialists attempt to explain how this can be so. The most common factor in such theories is the attempt to explicate the nature of mind and consciousness in terms of their ability to directly or indirectly modify behaviour, but there are versions of materialism that try to tie the mental to the physical without explicitly explaining the mental in terms of its behaviour-modifying role. The latter are often grouped together under the label ‘non-reductive physicalism’, though this label is itself rendered elusive because of the controversial nature of the term ‘reduction’.