"Affordances were characterised by Gibson as properties of the environment taken relative to an animal. Gibson emphasised the mutuality or reciprocity of the relations between animals and their environments but his own work was concerned primarily with the Gibson’s affordances 5 analysis of environmental information. Other ecological theorists have stressed the need to provide a complementary account of those properties of animals that enable them to act on affordances. The term “effectivity” has been widely used for this purpose and much of the formal work that has been done has had as a goal the elucidation of the relation between affordances and effectivities. Shaw, Turvey and their colleagues have argued that affordances and effectivities are “duals” of each other and the coalitional framework set out by Shaw & Turvey (1981) treats the concept of duality as fundamental. It is argued here that Turing’s computational model provides a clear account of effectivities and affordances and demonstrates the mutuality between them better than does the concept of duality. Gibson on Affordances. Gibson (1979, p.22) described psychology as “the study of the perception and behavior of animals and men as a function of what the environment affords.” He regarded the hypothesis that information in the ambient optic array specifies affordances to be “the culmination of ecological optics” (Gibson 1979, p.143). Affordances have many characteristics. They can be subsumed within a framework that focuses on the following seven features; affordances are ecological, they are relational, they are facts of the environment and behavior, sets of them constitute niches, they are meanings, they are invariant combinations of variables and they are perceived directly''.